It's not uncommon for trends in crafting and DIY projects to come and go. A "messenger bottle" is a term that could refer to a number of different types of projects, depending on the context. Some possible interpretations could include:
A small bottle or jar that has been decorated and filled with a message, trinket, or other small item, and then sealed or corked. The recipient of the bottle can then open it to reveal the contents.
A bottle or jar that has been transformed into a "messenger" of sorts by attaching a paper or fabric message to the outside of it. The message could be tied to the bottle with string, ribbon, or another type of fastener.
A bottle or jar that has been used to send a message over a long distance, either by physically transporting it or by floating it in a body of water. In this case, the bottle would likely be sealed and contain a message or other small item.
Without more information, it's difficult to say for sure what is meant by the term "messenger bottle." However, these are some possibilities that come to mind.