What is this pipe in my garage?
That looks like it is a wye pipe fitting (cleanout) with a cap on it, but with a hose added to it to help drain the pipe stack when it backs up. I can only assume that there is a partial blockage later in this pipe, and this helps prevent it from backing up into a sink upstairs. The solution is to remove the blockage with a roto rooter or equivalent, and replace the cap with one that doesn't have a hole in it.