What does "marginal probability" mean?

Probabilities may be marginal, joint or conditional. A marginal probability is the probability of a single event happening. It is not conditional on any other event occurring.

Some examples of marginal probability are the probability of choosing a red card from a deck of cards or the probability of choosing an ace from a deck of cards. Neither event relies on any other event. An example of a joint probability is the probability of choosing a red ace. This outcome requires the two events to occur jointly. A conditional probability is the probability of finding an ace on a red card that has already been chosen.