The magic lair a hidden lair where the Russo kids go to have magic lessons from their father Jerry. The lair can be accessed from the Waverly Sub Station via the freezer or from the Russo's apartment. The lair has a portal to the wizard world which can be used as an entrance for visitors.
Once Alex and her Uncle Kelbo turned into Sea Chimps and they had to figure out how to get them back into people and the lair needed to be in wateriness. Often, the freezer is used as a portal between the Waverly Sub Station, and the Russo home.
The lair is also were the Russos keep Kelbo's magical and dangerous (possibly) gifts to Therissa (so Alex doesn't play around with them)
Starting in the episode Alex Gives Up, Justin takes his delinquent class there.