“A titer is a laboratory test that measures the presence and amount of antibodies in blood. A titer may be used to prove immunity to disease. A blood sample is taken and tested.
If the test is positive (above a particular known value) the individual has immunity. If the test is negative (no immunity) or equivocal (not enough immunity) you need to be vaccinated.”
Reference
shs.gmu.edu/ufaqs/what-is-a-titer