An antivenom is a medicine used to protect against a specific venom. (A venom is a poison released by a snake or insect.) Frequently the venom of some creature such as a cobra will be harvested and a little will be injected using a hypodermic needle into a horse every day for a long time. The horse's body will develop an immunity to the venom, so more and more venom from the snake can be injected every day. After the horse is immune, its blood can be drawn little by little to get the part of the blood that makes the horse immune. That part can then be injected into humans who have been bitten by cobras.
There are antivenoms for the very large and very dangerous funnel-web spiders found in Australia, and for the widow spiders (such as the black widow) found all around the world.