P-Trap w/ Plug
A plumbing trap is designed to constantly hold some water. What that water does is to keep the sewer gas smells from escaping the drain.
P-trap also stops rodents and other small creatures from entering the house through the plumbing drain. In addition to catching substances from entering from the outside, the P-trap also catches hair, grease and other gunk that results from using household toiletries. However, these items can clog the P-trap and cause a slow drain. In this case, the P-trap should be removed and either cleaned or replaced.