CW stands for “Content Warning”. It’s a feature that allows users to hide their posts behind a warning message.
Some commonly used CWs include NSFW, Politics, Mental Health (Some times shortened to “MH”, often with a +/- depending on sentiment), Spoilers, Food, Alcohol/drugs, and flashing GIF/video.
The glitch-soc fork of Mastodon allows you to show or hide specific tags, yes.