An address is active when it holds a set minimum amount on the Polkadot and Kusama networks, this quantity is called the existential deposit (ED). In Polkadot the ED is currently 1 DOT, while on the Kusama network, it is set at 0.0000333333 KSM.
If wallet funds are lower than the existential deposit amount, the account will be deactivated (reaped) and any remaining funds are destroyed. In order to reactivate it, you must send an amount larger than the existential deposit. However, this will not restore the destroyed funds.