The unit of design means when you sit down to design a new program, you are basically deciding on a set of classes you will include.
The unit of reuse means if you want to reuse functionality someone else has written, the smallest unit you can pull into your new program without making a new copy of the code is an entire class. As Doc Brown pointed out, often because of coupling that unit is even larger. You can't reuse just one method, or just the data structure without any methods, as is common in functional and procedural paradigms.
Most OO programmers don't realize how big of a problem this actually causes for reuse. Generally, classes must be carefully designed with reusability in mind, and mainly only library designers go to the trouble.
The smaller the unit of reuse, the smaller the probability that the unit is specific to a single application. That's why the most successful reused code uses very small, widely-accepted, general-purpose interfaces. For example, a simple
compare interface on a type allows that type to be reused in a wide range of existing code, from sorting algorithms to data structures like binary search trees. Note, however, that the reused code must have been initially designed in order to take advantage of that interface.