From Liquipedia StarCraft Brood War Wiki
Racepicking means that you, depending on what race your opponent is playing, chose a race to face it. This is opposed to the standard way of picking a race where you stick to your main race despite whatever race you are facing. The idea of racepicking is to avoid a certain matchup that you do not feel comfortable enough playing, or that you feel that you are stronger at with a different race than your normal one.
Racepicking is rarely seen anymore in the Korean progaming scene. However, when 2on2 was still part of the Proleague, players would sometimes racepick or even pick random in order to get the deadly Zerg/Zerg combination.