The debounce filtering reports a key/switch change directly, without any extra delay. After that the debounce logic will filter all further changes, until the key/switch reports the same state for the given count of scans.
So a perfect switch will get a short debounce period and a bad key will get a much longer debounce period. The result is an adaptive debouncing period for each switch.
This value defines how often the same key/switch state has to be detected in successive reads until the next key state can be reported.
In other words this value defines the minimum debouncing period for a switch.