a practice girl is usually a fat/larger/ugly teenage girl whom teenage boys engage in sexual intercourse with for practice/expierence.
Practice girls don't have much social pull, nor does their pleasure matter, so they are perfect specimens for teenage boys to use to "step up" their sexual ability when they practice sex with a better/more attractive girl.
Guy A: "dude, i have a date with Jenn tomorrow night, she is soooo hot! Its just.... i'm worried because i'm a virgin"
Guy B: "Oh dude, just go fuck emily tonight.... she'll be at the party"
Guy A: "No way dude, she's a fat fuck"
Guy B: "So fucking what pussy? She just a practice girl, she doesn't matter"
Guy A: "Niiiiiice"