A granthi is the custodian and reader of the Guru Granth Sahib, and because Sikhs regard the scripture as the living Guru, that framing determines the whole character of the office.
Attending the scripture
The granthi reads from the Guru Granth Sahib in the gurdwara, leads the congregation in the recitation and the daily ceremonial care of the volume, its installation in the morning and its putting to rest at night, and attends to it as one serves a living and honoured Guru. The office is therefore one of service and custody rather than priesthood: the granthi does not stand between the worshipper and God, since Sikhism has no priestly caste, but serves the scripture and the congregation as a reader and caretaker.
Not a priest
There is no ordination and no sacerdotal order, and in principle any competent and observant Sikh who can read the scripture properly may perform the functions of the granthi, so the office rests on knowledge and good standing rather than on a conferred sacred status, and larger gurdwaras employ granthis while smaller congregations may share the duties. The granthi commonly also leads ceremonies and gives instruction, and in practice serves much of the function a clergyman serves in other traditions, but the tradition is careful to insist that this is a role of service to the Guru Granth Sahib and the community, not a priesthood, in keeping with the Sikh rejection of a mediating clerical class.