When something "looms" over you, it means that it's floating or hanging ominously. It feels threatening and makes you worry that something bad is going to happen. The best example of this is when dark clouds are floating in the sky. You can describe it this way if you write a story about a scene:
There were dark clouds looming overhead.
People also use "looming over" to talk about a bad event that they know is going to happen in the future:
My 40th birthday is looming right around the corner. I'm not looking forward to that!