In every one of the fixtures in your house connected to an appliance, you are likely to find a noticeably curved pipe called a "plumbing trap." The reason for this curvature in the pipes is to prevent nasty odors that develop in your pipes or that come up from the sewers from entering your house. In most cases, these traps will feature a "U" or "S" design that use the variations in water elevation as well as water pressure to prevent these gases from escaping.
For appliances like the kitchen or bathroom sink, the piping will come out of the wall horizontally, make the "U" curve and then continuing a vertical ascent towards the fixture outlet pipe. In general, the depth of the "U" shape should be at least two inches or so from the trap arm, also known as a fixture drain.
The trap seal is located in this first dip. Some experts recommend that the depth not exceed four inches, though it depends on who you speak to. An experienced Washington DC plumbing service like John C. Flood can verify for you what depth is appropriate given your appliance.
The way these traps work is that water gets stuck in these dips, essentially blocking gases from going up the "U" and out into your sink. The trap seal will go from the crown weir, which is the bottom side of the outlet pipe extending from your wall, to the trap dip, also known as an upper dip, where the top of the curved pipe is at its lowest point in the curve.
Along with preventing sewer gasses from backing up into your home, the trap will catch any debris with the potential to clog the home's plumbing from reaching a point in the pipes that can't be easily addressed by an experienced DC plumber.