Hiding is the act of remaining unseen and undetected in order to prevent getting killed.
When: If you're severely outnumbered and/or low on HP and/or ammo.
What: Staying out of sight.
Why: Forcing a draw instead of losing the round.
How: See hiding spots per map.
Bonus: Use hiding to ambush/surprise attack, rather than just hide.
How to practice: See hiding spots per map. Once you know where the spots are, go on an empty server and find best routes to those spots.