Compost is organic matter like orange peels and coffee grinds mixed together and left to decay, usually in a compost pile, then used as fertilizer. To make such a mixture is to compost.
To compost is to make a mixture of organic matter, such as manure and leftover food, and leave it somewhere like a compost pile in the back yard or in a bucket with a tight lid in the kitchen. The mixture is also called compost, and after it sits and decays for a while, voila! It turns into fertilizer that makes the soil rich and the plants happy.