Hi and welcome to the forum!
In the wild, RES need to be able to climb out of the water onto a dry area where they can sit in the sun for several hours. This is what is known as "basking". It gives them a chance to dry off completely which helps prevent fungus and shell rot. Basking also gives them a chance to warm their bodies up and also digest their food. Another very important reason for basking is so they can absorb the ultraviolet rays from the sun (both UVA and UVB rays). The UVA/UVB rays help turtles (and many other animals) produce vitamin D3 which is needed to make use of the calcium they eat to build their bones and shell. Without it they will develop deformed shells and a weak skeleton. It can also lead to death.
In our attempt to keep RES in captivity, we need to try to recreate their natural habitat as closely as possible. That means as large a tank as possible with as much water as possible (without letting them climb out and escape). It also means having a dry platform ("basking dock") for the turtle to easily climb onto and bask. Since we can't bring sunlight into the house (UV rays won't go through glass) we need to provide special UV lighting. You can get special light bulbs that provide UVA and UVB from the reptile section of most pet stores. Just make sure it says UVB (that's the most important one and because all UVB bulbs in the pet market produce UVA too). You will also need a regular lightbulb that will give off some heat. You need to have both of these lights shining right on the basking dock. The heat that the turtle should feel when he's on the basking dock should be between 88 and 90 degrees (F). The best way to measure this is to put a regular thermometer on the basking dock with the lights on and measure the temperature. If it's too high, use a lower wattage heat bulb or raise the bulb a little higher. If it's too low, use a higher wattage bulb or move it a little closer. Just make sure the bulbs are no closer than 8 inches from the turtle. Also follow the directions for correct placement of the UVA/UVB bulb.
