It is the plural form of the pronoun "I" (mimi). It can be used to refer to:
* The speaker and one or more other people: "Sisi tunaenda dukani" (We are going to the shop).
* A group of people including the speaker: "Sisi ni watoto wa Mungu" (We are children of God).
It is important to note that "sisi" is used for both inclusive and exclusive "we". There is no separate word for "we (including you)" and "we (excluding you)" in Swahili. The context will usually make it clear which meaning is intended.