Something described as lone is the only one of its kind — it's on its own. The lone child waiting for a ride home from school is the very last kid to be picked up.
The lone tree in your yard is the only one growing there, and if your well is your lone source of water, it's the only way that water flows from the shower or the kitchen faucet. Sometimes lone is used to mean "lacking support," as when a state senator's vote is a lone voice in opposition to a proposed bill. If someone calls you a "lone wolf," they see you as someone who prefers acting alone.