Timelapse is a technique whereby a series of still images is taken with a relatively long time interval between each image.
The resultant series of images is played back at a normal video frame rate.
This results in a speeded-up video.
This technique is a wonderful way of revealing motion and changes that normally progress too slowly for us to notice. Mist rolling across a scene; flowers opening; shadows moving with the progression of the sun across the sky; stars seemingly arching across the sky as the planet turns, etc. etc.