A Gaming Glossary: %

In general, to 100% a game is to fully complete it by finishing all levels, defeating all bosses, collecting all items, etc.
I just didn’t have the patience to 100% Fire Emblem: Three Houses.

100% can also be used as an adjective to denote a playthrough or run that is complete in this way.
A 100% playthrough of Skyrim can take more than 200 hours.

As a speedrunning category, 100% refers to full completion of the game (cf. Any%). What counts as 100% is defined by the community for a given game but often involves completing all levels, defeating all bosses, and/or collecting all items. The Super Mario Bros. 3 community, for instance, bases its 100% category on “completing every stage;” Legend of Zelda 100% is based on collecting items and defeating the final boss.

Some games that track percent completion internally might not have a “100%” category as such. For example, Castlevania: Symphony of the Night tracks how much of the map the player has uncovered; even without glitches, it is possible to exceed 100%. Thus, “100%” would be an unusual way to define a category for that game. On the other hand, some games have a category that is effectively 100% but is named in a way specific to that game. Thus, Super Mario 64 has a “120 Stars” category.
%In speedrunning and other forms of challenge gaming, “%” is as a suffix to describe the requirements for completing a game in a given category. Thus, common categories include “100%”, “Any%”, and “Low%”. The usage is sometimes extended to refer to milestones that are not really percentage-based (like the “Package%” category in Satisfactory) or even to refer to constraints on how the game is to be completed (Super Mario Odyssey‘s “Talkatoo%” category).