A water bath is a very common piece of laboratory equipment. It’s exactly what it sounds like - a container full of water. It’s used to incubate samples at a constant temperature over a long period of time. It’s often used to warm up reagents (like cell culture media) from fridge temperature (4 C) to either room temperature (23 C) or body temperature (37 C), depending on what the reagent will be used for. By submerging the reagents in water, they warm up faster than they would by just sitting on a bench, and the temperature can be adjusted as desired.