The KerbalX API mod (KXAPI) is a mod that enables other (authorized) mods to interact with KerbalX's API.
KXAPI enables someone to log into their KerbalX account from a KSP mod. KXAPI handles all aspects of user authentication, so mods that connect to the KerbalX API don't ever need to collect login credentials.
If your mod is granted access you (the mod developer and any development collaborators you select) will be given the rights to digitally sign new versions of your mod with the API. Only mod releases which have been signed will be able to access the API. If someone forks your mod and modifies it, that modified version will not be able to access the API.