An unglazed clay jar is porous, so tea stored in it exchanges a little air with the room. For pressed pu'er that is the point: a cake is broken up and left in the jar for a few weeks so it re-oxygenates and settles before brewing. The practice is called waking the tea.
Because the clay is porous it also absorbs whatever is nearby, so keep it away from coffee, spices and cleaning products. The lid is ground to fit and finished with a bead of seal so it closes with slight resistance.