Aground describes a boat that's accidentally gone ashore, or is stuck on the bottom of a lake or other body of water. If your kayak goes aground, you may need to get out and push it further out in the bay.
When you use the word aground, you're almost certainly talking about a boat or ship. If a sailboat runs aground, its hull can be damaged, which might even cause the boat to take on water. Running aground can be a minor inconvenience, or a major accident. Since about 1500, the adjective aground has been a nautical term that generally means "stranded on land."