It's when you get immune to getting infected by a virus - antibodies neutralize the virus particles before they can do any harm or meaningfully replicate. All current (and probably future) SARS-CoV-2 vaccines don't do this. Through a combination of several vaccinations and infections the hope is that you get close to sterilizing immunity.
If the vaccines are creating sterilizing immunity, this might be true. For example the measles vaccine does that.
But for all current SARS-CoV-2 vaccines we have we don't get sterilizing immunity.
So it can still make sense to get vaccinated even if society at large is vaccinated.