Any neighborhood, public, municipal, or otherwise not-in-your-backyard pool will tend to attract a diverse clientele, from the neighborhood elementary-schoolers to fitness-buff yuppies to cruising adolescents to the retired nurse with her two-year-old grandson. In terms of the manner in which they interact with the pool, however, these patrons can mostly be divided into two categories: adults and kids. Kids, which in this sense covers those from barely ambulatory age to somewhere in the middle of adolescence, usually spend the majority of their time in the water, hanging out with friends, splashing, playing games, or just swimming from place to place as the spirit moves them. The adults often tend to spend more time dry, sunbathing or soaking their legs, but when they do get in the water, they tend to do so with an agenda, swimming laps up the length of the pool and down again, over and over.
The problem we face, then, is that given a limited pool volume and/or the absence of multiple pools dedicated to specific functions, it is often difficult to accommodate both of these desires simultaneously - the kids get in the adults' way, the adults will intrude upon the kids' territory, and everyone ends up somewhat annoyed with each other. Thus the institution of adult swim. Adult swim is a period of time in which "adult" activities are formally given priority in a pool. This may take the form of restricting usage to lap swimming in all or part of the water, excluding all swimmers under a given age from the pool for the duration, or some similar mechanism. This doesn't entirely preclude all inconvenience or resentment - as a youngster, I got pretty ticked that I'd periodically get kicked out of the water and have to settle for playing suicide against the machine room wall - but in hindsight it's a pretty fair compromise, as the rest of the time the chaos of the pool tended to effectively grant control to the kids. Growing up, our pool's adult swim periods were the first fifteen minutes of every hour, plus six to seven PM, but obviously its times, or even existence, will vary with the pool, its demographics, the day of the week, the lifeguard on duty, the presence or absence of actual adults, and the like.