Silverado wouldn't turn on

Im a lawn care worker or whatever. Yesterday I went to HEB Plus to buy some powerades and water bottles. When I got to the truck and tried turning it on, it wouldn't.

Before that, the truck was working perfectly fine. Whenever I would turn the key, there was just a click. AC and stereo would turn on and all but the engine wouldn't. Someone helped me jump start the battery but it wouldn't work.

Thankfully I had my socket wrench and i could move things around. I unplugged the batrery and plugged it back in twice and nothing changed.

Third time I did, I cleaned the grabber which wasn't really dirty but that made it turn on. Battery life was a bit under half power.

Help me understand what really made it turn on. I feel like what I did wasn't really the reason it ended up starting the engine.