A system is "frozen" when, while still turned on, it does no longer react on any input.
The entire device might temporarily or completely freeze when e.g. some process is blocking a ressource. In some cases, this block is resolving automatically after a (short) while, in other cases the only remedy is to take out the battery (or toggle the reset switch on devices without removable battery).
There can be several causes to this, including "buggy drivers" as well as hardware issues.
Don't confuse this with the "ANR" (Application Not Responding) error, which usually only affects the corresponding app, while the device as such remains responding.