In the translation by Hill & Scarpitti:

It is unworthy for a philosopher to say that the good and the beautiful are one; if he goes so far as to add 'and also the trye', we should thrash him. Truth is ugly: we have art lest we perish from the truth (465).

The best gloss I'm aware of on this is Philip J. Kain's:

The truth is not good for human beings - the truth is horror. Reality as it truly is, is not beautiful - it is terrifying. To pursue the truth, far from pursuing the good and achieving happiness, as most all philosophers have assumed, would have the consequence of plunging humankind into the abyss, of rubbing their noses in the horror of existence. Life requires lies, illusion, art, veiling. Life must shun the truth. Life is not possible with the truth. To pursue the good, what is best for human beings, requires rejection of the true. (Kain: 47.)

One might protest, but surely it's always good to know the truth - to know what is bad, harmful, dangerous, for instance. How else but by knowing the truth can we avoid such things? Doesn't inquiry - scientific, historical, &. - aim at discovering the truth. If not, what is the point of it?

I'm not so sure about this - there are some truths I'd prefer not to know: the date of my death, for instance. But Nietzsche wipes away such protests in a wider perspective. Again Kain succinctly captures his viewpoint :

If the truth is that existence is horrible [which it is: GT], if the true is the furthest thing from the good [which is also is: GT], then at least since Socrates we have been involved in serious contradiction. Insofar as we pursue the real truth, insofar as we approach the horror and meaninglessness of existence, we are not headed toward meaning, purpose, or the good at all. We progress toward meaninglessness. On the other hand, insofar as we seek meaning, purpose, and the good, we must mask the true, conceal it, create illusion. What emerges from this are two different conceptions of truth, the truth as correspondence to reality and a truth which requires illusion, that is, merely, what we take to be true. We must recognize that Nietzsche has and needs both of these conceptions and that to understand him we must explore both. (Kain: 47-8.)

References

Philip J. Kain, 'Nietzsche, Truth, and the Horror of Existence', History of Philosophy Quarterly , Jan., 2006, Vol. 23, No. 1 (Jan., 2006), pp. 41- 58.

F. Nietzsche, The Will to Power [Der Wille zur Macht], tr. R.Kevin Hill & Michael A. Scarpitti, London: Penguin Random House UK, 2017.

In the translation by Hill & Scarpitti:

It is unworthy for a philosopher to say that the good and the beautiful are one; if he goes so far as to add 'and also the trye', we should thrash him. Truth is ugly: we have art lest we perish from the truth (465).

The best gloss I'm aware of on this is Philip J. Kain's:

The truth is not good for human beings - the truth is horror. Reality as it truly is, is not beautiful - it is terrifying. To pursue the truth, far from pursuing the good and achieving happiness, as most all philosophers have assumed, would have the consequence of plunging humankind into the abyss, of rubbing their noses in the horror of existence. Life requires lies, illusion, art, veiling. Life must shun the truth. Life is not possible with the truth. To pursue the good, what is best for human beings, requires rejection of the true. (Kain: 47.)

One might protest, but surely it's always good to know the truth - to know what is bad, harmful, dangerous, for instance. How else but by knowing the truth can we avoid such things? Doesn't inquiry - scientific, historical, &. - aim at discovering the truth. If not, what is the point of it?

I'm not so sure about this - there are some truths I'd prefer not to know: the date of my death, for instance. But Nietzsche wipes away such protests in a wider perspective. Again Kain succinctly captures his viewpoint :

If the truth is that existence is horrible [which it is: GT], if the true is the furthest thing from the good [which is also is: GT], then at least since Socrates we have been involved in serious contradiction. Insofar as we pursue the real truth, insofar as we approach the horror and meaninglessness of existence, we are not headed toward meaning, purpose, or the good at all. We progress toward meaninglessness. On the other hand, insofar as we seek meaning, purpose, and the good, we must mask the true, conceal it, create illusion. What emerges from this are two different conceptions of truth, the truth as correspondence to reality and a truth which requires illusion, that is, merely, what we take to be true. We must recognize that Nietzsche has and needs both of these conceptions and that to understand him we must explore both. (Kain: 47-8.)

References

Philip J. Kain, 'Nietzsche, Truth, and the Horror of Existence', History of Philosophy Quarterly , Jan., 2006, Vol. 23, No. 1 (Jan., 2006), pp. 41- 58.

F. Nietzsche, The Will to Power [Der Wille zur Macht], tr. R.Kevin Hill & Michael A. Scarpitti, London: Penguin Random House UK, 2017.