Typically a mesh is a set of proxies that are colocated with each service you deploy. The proxies facilitate service discovery and secure communication with other services. They abstract this functionality such that each service doesn't have to individually (and poorly) reinvent that wheel.
Your service would connect to database:5432, but instead of going straight to the database it goes to a proxy and the proxy has all the smarts to find and connect to the database, even if it's in another data center. The proxy can also do it securely such that you can go over the public network if necessary.
There's a good chance you don't need this complexity, but if you are hosting 100's of small services across multiple cloud providers, regions, or data centers, this starts to become an attractive option.
Your service would connect to database:5432, but instead of going straight to the database it goes to a proxy and the proxy has all the smarts to find and connect to the database, even if it's in another data center. The proxy can also do it securely such that you can go over the public network if necessary.
There's a good chance you don't need this complexity, but if you are hosting 100's of small services across multiple cloud providers, regions, or data centers, this starts to become an attractive option.