I know this is me because it happens…

I know this is me, because it happens with EVERY sewing machine I’ve ever used.
I start sewing, then something feels like it jumps and all of a sudden I get this bundle of thread at the bottom.
I’ve sewn an entire dress to the very end and now I keep getting this and I can’t make it stop.
I’m pretty sure it has something to do with the bottom thread, which I think is called the bobbin, but I can’t figure out what.
What am I doing wrong? Do I have to tighten something?