A typical network goes like this:

Cell phone > cell tower > ISP > internet

Mesh goes like this:

Cellphone wifi A > cell phone wifi B > cell phone wifi C (or Bluetooth or other ad hoc connections

Or (for example)

Cellphone A > Cellphone B > Cellphone C > cell tower or wifi > internet

It's basically chaining devices together so they can communicate. Very similar to a regular network, but reliant on individual devices rather than something centralized like a cell tower