A mobile virtual network operator (MVNO) is a telecoms company that provides mobile phone telecoms without owning its own network. MVNOs essentially buy capacity in bulk and resell it retail.
From a customer's point of view the MVNO is their mobile phone network operator as they pay the MVNO. The MVNO will usually provide some services, but the calls are actually carried on a network belonging to someone else.
An MVNO does not have the heavy capex commitments that owning a network necessitates; this also means that barriers to entry are low.