Something that's harebrained is reckless or foolish. Sometimes, harebrained ideas work out just fine — several people have survived trips over Niagara Falls in a barrel, for example.
If you have what you think is a great idea, but someone else calls it a harebrained scheme, it means they think you'd be a fool to try it. For example, your harebrained plan to ride your bike on the highway is bound to get you in trouble. The 16th century harebrained came from the earlier harebrain, "giddy or reckless person," from the common impression of a rabbit or a hare as being nervous and skittish.