A unit block is an AutoCAD block definition in which all of the graphics are drawn within the confines of a one-unit by one-unit area (or, for 3D blocks, a one-unit by one-unit by one-unit volume). The block can then be inserted with X- and Y-scale factors (or X-, Y- and Z- scale factors in the case of a 3D block) equal to the actual size of that instance. For example, HVAC supply diffusers are often represented by a rectangle with opposite corners connected by diagonal lines
You could set up a number of blocks for all of the different sizes you would need (or, in more recent versions, master the creation of dynamic blocks). Or, you can create a single, unit block and then set the X- and Y- scale values to get the size needed for a particular instance.
The unit block concept remains useful, even with the advent of dynamic blocks, as ACA objects that use blocks (such as for the view blocks of Multi-View Blocks or for custom display blocks added to many object types) often can take advantage of the unit block concept, and dynamic blocks will not remain dynamic when nested within an ACA object.