A dip tube is the long plastic tube inside a tank water heater that carries incoming cold water down to the bottom, where the burner or element heats it. This keeps cold water from mixing with the hot water waiting at the top, so you get full hot-water output until the tank runs low.
A dip tube is a simple part that makes a tank water heater work correctly. It is a long tube, usually plastic, that runs from the cold water inlet at the top of the tank almost all the way down to the bottom. Its whole job is to send incoming cold water to the bottom of the tank instead of letting it dump in at the top.
The reason that matters comes down to how a tank heater stores hot water. The burner or heating element sits at the bottom of the tank. Heated water rises, so the hottest water collects at the top, which is exactly where the outlet draws from when you open a hot tap. If cold water entering the tank were allowed to mix in at the top, it would water down that hot supply immediately. The dip tube prevents this. It carries the cold water down to the bottom, where it gets heated and then rises in its turn. This layering of hot water on top of cold is called stratification, and the dip tube is what keeps it working.
The tube stops a few inches short of the very bottom, typically around 8 inches up. That gap keeps the incoming water from stirring up the layer of sediment that settles on the tank floor, so the sediment stays put instead of getting pushed toward the outlet.
A dip tube can crack, break off, or, on some older units, slowly disintegrate. When it fails, cold water pours in at the top and mixes straight into the hot supply. For example, a household that suddenly gets only lukewarm showers, or runs out of hot water far faster than before, may have a broken dip tube rather than a failing heating element. On units where the tube crumbled, bits of white plastic can also show up in faucet aerators and screens. A dip tube is inexpensive and replaceable, so this is often a cheap fix for what looks like a dying water heater.