If there are any protrusions on the spider's abdomen, these are the silk glands.

An excerpt from the article 28 facts about spiders

They are used to spinning silk thread from which spiders build their webs. Most often, spiders have six silk glands, but there are species with only one, two, four, or eight. Silk nets can be used not only to build webs but also to transfer sperm, build cocoons for eggs, wrap prey, and even build balloons/parachutes so they can fly.