An input is a space on a screen where someone can enter a piece of information. It might ask for a name, an email address, or a search word. Think of a blank line on a paper form: you fill in the part that is missing.
Different inputs can be suited to different kinds of answers. A password field hides what you type, while a date field can help you choose a day. The label next to the field should make it clear what information belongs there.
For example, a sign-up page might have one input labeled “Email address.” Use a short, specific label so people do not have to guess what to enter.