A resonant head is the name for the head on the bottom of a tom tom. The head is not struck but vibrates in sympathy with the top head. It is very important to tune the batter and resonant heads to work with each other.
Standard drum heads are acceptable for the resonant side however it is common to see a lighter weight head used. Used heads are unsuitable for the resonant side.
A snare drum uses a very different resonant head made very thin in order to develop more greater response and sensitivity to the vibrations caused by hitting the drum's batter head.
Please visit the Snare Side head page for specifics