An attribute is a slot in a node that can hold a value or a connection to another node. Attributes control how a node works. For example, a transform node has attributes for the amount of rotation in X, Y, and Z. You can set attributes to control practically every aspect of your animation.
Attributes are similar to variables: they hold some value of a certain data type. The difference is that usually, setting the value of an attribute will cause some aspect of the scene to be recomputed. For example, changing the rotateX attribute of a transform node rotates its object.