The term, basically, boils down to this: If you have a mail-in ballot, someone comes to your house and collects it for you and later submits it with bunches of other ballots, collected in the same way - "harvesting."
It is legal for third-party groups - maybe a community organization, a voter advocacy group, maybe affiliates of the Democratic or Republican parties - to do this in some states.
But is it legal in Georgia? Short answer: It is not.
The bill H.B. 316 passed in 2019 by the Georgia General Assembly and signed by Gov. Brian Kemp in April that year made a number of changes to state election law, among them specifically regulating against the "harvesting" practice.
It changed the law to specify that either the voter "personally mail" the ballot or have it delivered by "the elector's mother, father, grandparent, aunt, uncle, brother, sister, spouse, son, daughter, niece, nephew, grandchild, son-in-law, daughter-in-law, mother-in-law, father-in-law, brother-in-law, sister-in-law or an individual residing in the household of such elector."
Pretty much, any close family member or someone who otherwise lives with you.
It also added that any disabled elector may have it delivered by their caregiver, and if a voter is in jail may have it delivered by an employee of the jail.
If someone does not meet those criteria, and they try to submit a mail a ballot on behalf of someone, they're in violation of Georgia law and subject to punishment.