What was the hiding place probably sufficient for?

The hiding place was sufficient for briefly hiding visitors from the Gestapo during a search.

On hands and knees Betsie and I crawled into the narrow room behind it. Once inside we could stand up, sit, or even stretch out one at a time on the single mattress. A concealed vent, cunningly let into the real wall, allowed air to enter from
outside.
“Keep a water jug there,” said Mr. Smit, crawling in behind us. “Change the water once a week. Hardtack and vitamins keep indefinitely. Anytime there is anyone in the house whose presence is unofficial, all possessions except the clothes actually on his back must be stored in here.”