overlay=25:25 means we want to position the image 25px to the right and 25px down, originating from the top left corner (0:0).
enable='between(t,0,20)' means we want the image to show between second 0 and 20.
[0:v][1:v] means that we want the first video file we import with
-i, in our case input.mp4 or how ffmpeg sees it video input file number 0, to be under video input file 1, in our case image.png.
:v just means we want video 0 and video 1.
[0:a] would mean we want the first imported audio track. Which would also come from input.mp4 but would point to the audio track instead of the video track in the mp4 file.