traying means discharging your batteries on a discharge/equalizing tray. Even if your charger has a discharge function it will discharge the entire pack to whatever voltage you set it to (5.4 volts if using a 6 cell pack) instead of each individual cell like what a tray will do.
The tray will get each cell down to 0.0 if using a deadshort tray made by Integy, Much More, Novak etc. Novak also makes a smart tray that allows you to set the discharge cutoff between 0.9, and dead short.
The advantages of a tray is your batteries will last longer because a tray almost acts as a conditioner for your batteries.
I'm tired and a little drunk here having a hard time explaining this. Someone kick in here please.