Inside the sewers that your pipes drain into, there are all kinds of smells and gasses. This is referred to as sewer gas, and it could actually harm your health if you were exposed to it. To keep this gas from getting into our homes, there is a pipe designed to block it from backing up called a p-trap. You will find this type of pipe underneath any sinks you have in your home. Many appliances have one built into them, as does your toilet.
In a p-trap, there will be water sitting inside of the u shaped bend, cutting off access for the gas.
Sometimes, though, p-traps don’t do their job properly, and this can be due to a number of reasons. For example, if you connect more than two sinks and/or appliances to a single p-trap, it could become overloaded and back up. It is also possible that the water evaporates from a p-trap that is used infrequently. To resolve this you might need to install a tap primer.