The term fast can have several meanings depending on its context.
1) In a general sense, it means happening or occurring quickly; moving with great speed. For example, "He ran so fast that he left everyone behind."
2) It can also refer to abstaining from food and drink for a period of time as in the religious observance of Lent where people may give up something they enjoy for 40 days. In this context, it means going without or denying oneself something. For example, "During lent, I am fasting from meat."
3) It can also mean an immediate response to a question or request as in the phrase "Can you please fast-track my application?" which means to give priority treatment to speed up the process of approval. In this context, it refers to quick action or promptness.