There is a common metaphor in the suttas used for all kinds of issues (sila, mindfulness, etc.) about a man carrying a bowl of oil. The idea is that you are carrying a very full bowl of oil through a crowd of people. The crowd is watching dancing girls. And behind you is a man with a sword. If you spill even a drop, he’ll cut off your head. It’s a metaphor for the kind of attention that is required in meditation.
I actually had this experience recently. I was at my parents house, and I had been cooking breakfast for everyone, and there was a full paper bowl of bacon drippings. I started carrying it out to the trash, and I realized, HA! here I am! The kids were playing in the living room, and I was carrying gross bacon grease across the carpet to the garage. Just a little move this way, and bam, disaster. Nobody was going to chop off my head, but it would be pretty bad.
Feeling what that was like was actually pretty amazing. I needed a lot of focus, or it would spill. But I couldn’t be too tight about it either. After all, I had to walk, watch out for walls and such as well as carry the oil.
It’s very much like learning any skill. Like say learning to golf. The instruction for how to hold a golf club is to hold it like you would a live bird. You have to hold pretty hard or it will fly away. But too hard, and, well, yuck. It’s really very similar to how tightly we have to hold our concentration in meditation. Too loose and it flies off. Too tight and, yuck.
When I started meditating, holding concentration too tightly didn’t really seem like a problem. But it can be! Trying to hold the focus too tightly gives you headaches, it’s no fun, and ultimately you stop making progress.