Off-lane heroes typically have sustain, mobility, wave clear, or all three. During the early phases of a game, the off-lane hero will solo the lane farthest from the other two (meaning said hero will be alone in that lane). Ideally the hero will be able to sustain themselves in the off-lane allowing the rest of their team to deal with the others. On certain maps, the rest of the team may also have the opportunity to split push mid and bot (or top and mid) as four.
Please let me know if I have forgotten any heroes - its a lengthy list!