A frame bag is a small item of cycle luggage attached to the front or rear of the top tube. It is useful for carrying small items that need to be readily accessible during the journey, such as keys, wallet and phone.
Some designs of frame bag hang on the top tube, like a tiny pair of panniers, strapped to the head tube. These are very stable but they are not compatible with down tube shifters because they get in the way.
Other designs of frame bag sit on top of the top tube, strapped to the stem at the front. These can flop to one side when full.
The alternative is a triangular frame bag. This sits inside the front triangle, near the seat post. This design is very stable and offers plenty of capacity but it requires the rider to dismount in order to access the contents.