BGP advertises only the best path by default
BGP routers use a list of attributes to determine the best path. BGP routers will only advertise one single best path to their neighbors. When a better path is found, it replaces the current path. Advertising a path and replacing it with a new path is called an implicit withdraw.
Since we only advertise the best path, a lot of other possible paths are unknown to some of the routers. We call this path hiding.
This behavior can be changed using features such as BGP Multi-Path and BGP Additional Paths.