Dwell - the solenoid acts as a valve, supplying air to cycle the marker after a trigger pull. The dwell is the time this is left open to allow air to flow in. Measure in milliseconds, set it to high = inefficient, too low, not sufficient pressure to shoot consistently / fast enough
Debounce is a way of ensuring only trigger pulls fires a ball. Can be mechanical or electronic depending on marker, usually again based on time between trigger pulls
Generally I would stick with stock settings!