Two main definitions, really, one noun and one verb.
1. A founding document that describes the legal standing, structure, and goals of an enterprise. Colonies, towns, and schools typically have something like this. You can think of it as a certificate of permission that says you're allowed to build something and why.
2. Hiring a vessel to take you somewhere. Chartering passage on a boat is essentially buying a boat ride.
As used in Kingmaker its the (1) definition that WatersLethe mention.
Its also kind of similar to the "Letters of Marque" often used the Age of Sails/seafaring pirate contexts. If that helps in finding an appropriate translation/concept. Its a formal grant/delegation of power, and often one that's designed for public display/presentation, rather than a "secret" contract between the issuing person and the receiving person.
If someone asks you (your Kingmaker party) why are you doing this? What right do you have to be here? Saying (and showing) "We have a charter from Jalmandi Aldori to explore this land." would be a common reply. People may not acknowledge her ability to issue that permission, but they'd temper their response based on their feelings of her ability to enforce the charter.
This thread should be in the Kingmaker forum.
The charter in Kingmaker is a legal document, explaining that the party are in the Stolen Lands on a mission with sanction from Jamandi Aldori. It's what gives them any legitimacy in the eyes of Brevoy, rather than just being them bandits and squatters.