A Pension Fund Manager, or PFM, is one of several PFRDA-approved entities responsible for actually investing and managing the money contributed to your NPS account, based on the asset allocation choices you make (or the default auto-allocation model, if you don't choose). You get to pick your preferred PFM from an approved list when you open your NPS account.
Different PFMs may deliver somewhat different returns over time, based on their specific stock and bond selection within each asset class, even though they're all following the same broad regulatory framework. NPS allows you to switch your PFM a limited number of times per year if you're unsatisfied with performance, giving you some flexibility to change managers without having to exit and re-enter the scheme entirely.
Reviewing your chosen PFM's historical performance across different NPS asset classes periodically is a reasonable habit, much like reviewing any other long-term investment choice.