When the pipe goes down, and back up, and back down again, the first "up-curve" is called the trap because when the water goes down the drain, not all of it goes back up and down again (thanks to gravity).
So there is a little bit of water there and it prevents any gas/odors from coming back up the sewer pipe. Provided that the pressure on the sewer-side of the trap does not exceed atmospheric pressure on the drain-side, the water will not get forced back up.
Provided that the water is not allowed to evaporate, there will always be water there. And provided that there is enough water such that the top of the inside curve of the up-turn is lower than the top of the trapped water level, the trapped water will seal out the sewer gas.