pinch flat aka snake bites.

this is when the pressure in the tire is low enough and your tire tries to go over a rock or hit a curb and the tire goes squish and flattens all the way so that the tire the innertube are pinched between the ground and the rim of your wheel. If there is enough pressure on the tire and the innertube where it flattened completely, the rim will literally cut two sometimes one small holes in the innertube and will cause a flat. The rim of your wheel act as a knife. Not a good thing.

It's also referred as a snake bite because it generally leaves two puncture holes next to each other like it was bitten by a poisonous snake.

If you are losing pressure while riding, then you have a slow leak. Get it fixed.

smilingcat