Mechanics are aspects of the game that describe interactions rather than content. Rather than discrete entities like Warframes, weapons, factions, mods, or planets (to name a few of many more), this page contains articles about values, applications, physics, and other ideas that exist: damage, maneuvers, attributes, assassination marks, reloading, and the like.
Whether or not a topic is a mechanic can be ambiguous in some cases. ("Are Missions content because there are countably finite ones in the game you can enter and exit, or are they mechanics because they describe a game feature distinguished from being in the Liset which acts as an overworld menu?") When adding to this category, do your best.