to work to the point of exhaustion: to toil hard, and so to be worn out by the effort
This verb names hard, tiring work and the tiredness that comes with it. It is not the neutral word for doing a job; it carries the sweat and fatigue of effort that costs you something. The same word can point at the toiling and at the exhaustion it produces, which is why English translations move between "labor," "work hard," and "grow weary" depending on the verse.
Most of the time the New Testament uses it for strenuous work: a fisherman who has fished all night, a farmer working his field, a person earning an honest living with his hands. Alongside that everyday use, the apostle Paul takes the word up as his favorite description of Christian service. When he speaks of himself, of coworkers like Mary and Persis, and of church leaders as those who "labor" among the believers, he means demanding, self-spending work done for the sake of the gospel, not casual helping.
Because the word is built on the idea of wearying effort, it can also stop at the tiredness itself. In a few places it describes simply being worn out: Jesus tired from a journey, or a church commended for pressing on and not giving up. The core stays the same throughout: this is effort that drains you.