I'm new to Starcraft and when I look on the Starcraft subreddit or here in some questions I often encounter term RAX usually in context with Terrans. But I don't know what it means.
What does it mean?
"RAX" is shorthand for "barracks", a Terran structure that produces infantry units.
It's often used to describe build orders. So something like "3-Rax Reaper" is denoting an opener that involves getting three barracks quickly and producing Reaper units with them.