Welcome to your personal Hideout! For now it is an old bomb shelter clogged with rubbish, but by making an effort you can make it a full-fledged underground base with everything you need for comfortable living.
You can use items and resources you found in raids to build and improve various areas of the Hideout, many of which will give you special bonuses and abilities — increased rate of off-raid health regeneration, energy and hydration, boosted skills leveling, the ability to craft various items, etc.
Escaping from Tarkov can take quite a while, and you need a place where you can regroup and regain strength. This is the place — your Hideout.
Good luck!