Coffee is high in nitrogen(~2%) which makes it a good fertilizer for plants indoors and out. Used coffee grounds from your coffee pot filter also contain potassium ( >1%), phosphorous (~.3%) and other trace elements.
You can mix some grounds into the dirt when you repot your house plants. Or you can sprinkle some used grounds on the top of the soil of your house plants. Use them sparingly. The nitrogen is released quickly, not over time, and you do not wish to give your house plants fertilizer burn.
Plants that prefer acidic environments especially like to have some coffee grounds added. Also foliage plants ( non-flowering) do very well with coffee grounds.
It is better to let them dry out before adding them to your plants. If they are put on too thick while wet they can mold.