What Sartre has in mind is that every other being in nature has a developmental pattern intrinsic to it. It has an essential nature, or 'essence', and its nature fixes its future development. Acorns become oak trees. Lambs become sheep. Uniquely, human beings as persons or agents have no such inherent developmental pattern or essence. As Sartre slightly gnomically puts it, our existence precedes our essence. In other words, we exist and create our own nature; we decide on our own essence in the sense that there is nothing, no kind of person, that it is proper or natural for us to become. Religious views typically contradict this, of course : God creates us with the intention that we should pursue certain goals or values.
For Sartre there is no escaping this 'radical freedom' in choosing our own goals or values. Even if we take a religious view and hold that God has created us to pursue certain purposes, the adoption of this view is itself an example of radical freedom. We have created a self-image as a being that serves and is meant divine purposes. To do this is not to recognise and act on a truth but to exercise radical freedom in this particular direction. It is 'bad faith' - a denial of our radical freedom - to suppose otherwise.
We are 'condemned to be free'. This means that we are totally responsible for every action and non-action. We cannot lean back on any such excuse as that we are 'naturally' this or that way or that society has conditioned us to possess and exercise certain dispositions. We cannot rid ourselves of our radical freedom while we remain human beings. Life might be a lot easier if we could.
I don't endorse Sartre's views here, for all the superb psychological insight he shows elsewhere. I am giving an account of Sartre's meaning, so far as I understand it, and not endorsing this part of his view of human beings.
In particular, I can't square Sartre's account of human beings exercising their radical freedom as persons or agents with what behavioural genetics tells us about human beings as living organisms. The connection between the two accounts is studded with paradox.
Sartre could, of course, retort that the image of human beings created by behavioural genetics is yet another mask or image which we create in bad faith in yet another denial of radical freedom. I enter no farther into the maze.