It's a pronoun that can be used as the subject of a verb.
Here are some examples:
* Il est grand. (He is tall.)
* Il pleut. (It's raining.)
* Il mange un sandwich. (He is eating a sandwich.)
It's important to note that "il" can also be used as part of some common expressions, such as:
* Il y a... (There is/are...)
* Il faut... (It is necessary to...)
* Il fait... (It's... weather-wise)