A suggestion is an idea someone proposes. You can accept or reject a suggestion.
When someone gives an order — like in the military — it has to be carried out. On the other hand, a suggestion is an idea that's optional. At a restaurant, a waiter may make a suggestion of a good soup. A friend could make a suggestion of a fun movie. If you're new in town, you might ask for suggestions of good neighborhoods. Suggestions are ideas that can be taken or rejected; it’s up to the person who receives the suggestion.