Kitchen sink - loose tap
There should be a couple of threaded rods that are screwed into the body of the tap and stick out about an inch, pointing the same direction as the hoses. There should then be a rubber washer and a steel washer in a horseshoe shape with a couple of holes in them that slide onto the threaded rods from underneath the sink. When you tighten a nut on each of the rods, the washer clamps the tap onto the sink.

If yours are missing, you might search for a "monobloc tap fixing kit". Here's an example for illustration but you'll need to make sure you get one that fits your tap.