That’s an INGCO tool pouch / tool belt bag
What it’s used for:
It straps around your waist and keeps your most-used tools right on you so you’re not climbing up/down ladders or walking back to the toolbox every 2 minutes.
Common stuff people put in it:
1. Hand tools - Pliers, screwdrivers, tape measure, utility knife in the slots/pockets
2. Fasteners - Nails, screws, bolts, anchors in the big pockets
3. Small parts - Drill bits, markers, pencils, phone, keys in the side pockets
4. Hammer loop - The D-rings on the side are for hanging a hammer, tape measure, etc