To get to this page, go to Settings → Misc Settings → Bandwidth Pools.
With basic bandwidth limiting, in a Service's settings you can set a throughput limit per session - eg 10kB per second per session. With this limiting, it is difficult to limit the total bandwidth usage of VPOP3, because each session has its own limit.
With Bandwidth pools, you set how much bandwidth is available in a 'pool', eg 100kB per second, and then assign sessions to that pool. All the sessions assigned to that pool will share the assigned bandwidth amongst themselves, so as more sessions are started, the available bandwidth per session reduces.
See the Services → Bandwidth Throttling topic for more details.