What are some scientific concepts whose meaning many people get wrong?

Many people, perhaps having heard and vaguely understood what homeopathy professes to do, think that antivenins contain small doses of snake venom, which are given to a snake-bite victim. Their logic goes, "A little bit of snake venom cures their snake bite."

No.

Antivenins are made by injecting venom (that is milked from a snake) into horses and extracting the antibodies from the horse after it has become tolerant of the poison. The horse's antibodies are what neutralize the snake's venom.

The same method is used for tick paralysis and some spider bites.