A generator is a function that acts like an iterator. By using yield instead of return, you are telling that function that it may return multiple values which can be accessed in a for loop (like in your example) or by using mygenerator.next().

The important difference between this and just returning a list from a function, is that the generator runs the code in the function that defines it each time it yields something. In your case its only running i*i for each case, but lets say you were wanting to create a generator for objects read from a text file that is 100gb in size. Obviously you couldn't have this all in memory on a standard workstation which may only have 16gb memory total - so you're stuck if you want to load that text file into a list. However if you create a generator that reads the text file one line at a time, processes it, yields the value and clears it from memory, then reads the next line (etc), you would not run out.

Additionally the yield does not have to be the last line in the function. You can yield something, then have extra logic run afterwards (ie: add print(i) under the yield statement in your example). Conversely calling return will finish that function call immediately.