It may slip some viewers' minds at points while watching Toy Story, but (most) of the characters involved in it are, well, toys. With that being said, Lightyear takes a look at the same universe that Toy Story took place in, albeit from a different angle. In the new film, the story of the human Buzz Lightyear, the person whose commendable efforts as a space explorer made him a star with his own toys, is explored.
It may sound confusing, but the Buzz that fans will see in Lightyear is not a toy, he's an actual person (albeit animated, Pixar style). During the movie, fans take a look at Buzz's early days as an elite member of an unnamed space force jetting from planet to planet in order to save the galaxy. The trials, tribulations, and successes he encounters in the film serve as the basis for him becoming the legendary space ranger that he is known and immortalized as a toy as in the Toy Story storyline.