Delta = difference
The location is relative to the current location/rotation, not an absolute location/rotation
If you keyframe a rotation form 0 to 90 degrees normally. Rotate the object 45 degrees and when you play the animation it will still rotate from 0 to 90 degrees
If you keyframe a delta rotation from 0 to 90 degrees. Rotate the object 45 degrees, when you playback the animation it will rotate from 45 to 135 degrees (a 90 degree differences from the current state)
The delta transform values are in the Object properties
