A giani is one learned in Sikh scripture, and in ordinary use the person who explains it, the word meaning simply the knowledgeable one.
Learning and exposition
The giani function is to know the scripture deeply and to expound it, explaining the meaning of the hymns of the Guru Granth Sahib to the congregation, giving discourses, and instructing in the tradition, so the office is one of learning and teaching rather than of ritual or priesthood. The giani makes the scripture, much of it in older forms of language, intelligible to the community, and the discourse expounding a passage is a regular part of gurdwara life, so the office serves the understanding of the tradition.
Knowledge, not ordination
The standing of the giani rests on knowledge, acquired through study of the scripture and the tradition, sometimes at the traditional schools that train such scholars, and it is recognised by the community rather than conferred by ordination, in keeping with the Sikh rejection of a priestly caste. The title is also used more formally as a scholarly qualification in the study of the Sikh scriptures and language. The giani often serves alongside or overlaps with the granthi who reads and cares for the scripture and the ragi who sings it, the three together carrying much of the religious service of the gurdwara, and the particular contribution of the giani is the learning that opens the meaning of the scripture to those who hear it.