Incremental encoders output pulses each time (and only when) the shaft is turned (or linear target is moved).
Absolute encoders continuously report their position as a digital message of 1’s and 0’s, whether the encoder is moving or not. They remember their position even if power is turned off.
The analogy is similar to a clock—the ticking of a clock would be the incremental output, the hands would be the absolute output. By counting the ticks, it is possible to determine the amount of time, but you must have a starting point.
It is possible to replace an absolute with an incremental encoder, and vice versa; software and hardware changes to your system may be required.