If you're aimless, you don't have a destination or a goal in mind. Aimless wandering through a forest might take you in circles, or worse yet, get you lost.
The adjective aimless is good for describing things that have no particular purpose or plan. You might prefer traveling with a map or GPS, while your best friend likes to drive in an aimless way, turning down dirt roads or exiting the highway impulsively. When you call a person aimless, you probably mean that he doesn't appear to have any ambition: "My brother's an aimless drifter." Aimless comes from aim, "purpose" or "target."