In a pool made for competitive swimming, markings are put into place in each lane to help swimmers gauge where they are in the water. These markings are meant to contrast from the other tiles on the bottom of the pool so that swimmers can clearly identify where they are.
A line runs down the center of each lane. During practices, this line serves for swimmers to help circle swim, which is the term used to describe the proper position in a lane when multiple people are using it at once. Swimmers doing multiple laps will start on one side of the centerline, then once they reach the other end of the pool and turn, move to the other side of that line. This is considered proper etiquette in virtually any pool used for athletic purposes.
During competitions, when a swimmer is the only person in his or her lane for a race, swimming directly on top of the line is important. Swimmers who stray to one side of a lane or the other are inadvertently adding more distance to their race. Shifting by a meter over the course of many laps results in covering unnecessary distance and ultimately has a negative impact on the race time.
The line ends in a T approximately five feet from the wall. At the very end of each lane a plus-sign marking, also known as a target line, is typically centered on the wall. These two markings are used for swimmers to gauge how close they are to the wall so that they can properly execute a turn.
While these markings and lines are standard at a competitive aquatic facility, they're not universal. Depending on the length, depth and primary use for a pool, they may change. Some pools have a line to mark where the pool depth begins to change.