Attributes, for Aristotle, scholastics, Descartes, and Spinoza alike, are the non-accidental qualities/properties expressed in language by predicates, as substances are expressed in it by subjects, to which they are predicated. Taken together, they make a substance what it is, hence they are essential (unlike accidental properties), constitute its essence. Substances that do not share attributes have nothing in common, and the more attributes they have the more "real" (concrete) they are. God, as the ultimate reality, is the "substance of infinite attributes", but according to Spinoza "the human mind has an adequate knowledge of the eternal and infinite essence of God". Here are some examples from Part II of Ethics:
PROP. I. Thought is an attribute of God, or God is a thinking thing.
PROP. II. Extension is an attribute of God, or God is an extended thing.
A somewhat more idiosyncratic use of "attribute" by Spinoza corresponds to what we would call "aspect" or "perspective", in which a substance is unfolded or under which it is considered. In particular, God and Nature are the same substance considered under different attributes (which is perhaps why we have an adequate knowledge of it). Here are some examples, again from Part II:
PROP. V. The actual being of ideas owns God as its cause, only in so far as he is considered as a thinking thing, not in so far as he is unfolded in any other attribute...
Note to PROP. VII. For instance, a circle existing in nature, and the idea of a circle existing, which is also in God, are one and the same thing displayed through different attributes. Thus, whether we conceive nature under the attribute of extension, or under the attribute of thought, or under any other attribute, we shall find the same order, or one and the same chain of causes - that is, the same things following in either case. Thus, whether we conceive nature under the attribute of extension, or under the attribute of thought, or under any other attribute, we shall find the same order, or one and the same chain of causes - that is, the same things following in either case.
Several aspects of interpreting Spinoza's theory of attributes are controversial. For instance, he ultimately concludes, in contradistinction to all his predecessors, that there is but one substance, God=nature, and despite mentioning its infinite attributes names only two of them in Ethics, thought and extension. In the early Short Treatise those are the only two through which God can be known, and omnipotence, eternity, immutability, and infinity are termed "propria", because "they are only Adjectives, which cannot be understood without their Substantives. That is to say, without them God would indeed be no God, but still it is not they that constitute God ; for they reveal nothing of the character of a Substance, through which alone God exists". In Ethics the "propria" disappear altogether, but eternity appears in the oft-mentioned sub specie aeternitatis, "under the species of eternity", in a sense similar to "under the attribute":
Corollary II to PROP. XLIV. It is in the nature of reason to perceive things under a certain form of eternity (sub quâdam æternitatis specie).
SEP has a nice article on Spinoza's Theory of Attributes.