Brown signs on highways typically indicate:

Brown signs identify recreational, cultural, and historical destinations: national parks, state parks, monuments, historical markers, scenic viewpoints, museums, and similar attractions. The brown color sets them apart from regular navigation signs (green) and motorist services (blue), so a driver looking for a park can scan for brown signs specifically. Brown sign destinations are often less obvious in the landscape than highway exits, so they get this dedicated category to help travelers find them. The standardization is part of the MUTCD; brown signs work the same way in every state.