Ableton's Strum transformation takes a simple chord clip and makes it feel like a real guitar take. By offsetting the individual notes in sequence, it creates that characteristic roll you'd get from a strummed string instrument, rather than all notes hitting at once.
The Tension slider shapes the timing curve of the strum, so you can dial in whether it feels loose and relaxed or quick and snappy at the start. It's a small but effective way to add physical realism to programmed guitar parts without recording a single note.