Plethora means an abundance or excess of something. If you have 15 different people who want to take you on a date, you have a plethora of romantic possibilities.
Plethora comes from the Greek for "fullness." Although it was originally used only in old-fashioned medicine to describe the condition of having too much blood, we use it to talk about any excessive supply. If you run a theater and all the seats are taken, that's a full house. But if the seats are full and people are standing in the aisles, you have a plethora of patrons. The stress is on the first syllable: PLETH-uh-ruh.