What does "getting titers done" mean

I've heard it before, so I looked it up...but can somebody explain it to me in simple English?

As others have indicated, blood titres are done to determine the degree of antibodies toward specific diseases. They are used to determine, for example, if the exisiting antibodies are strong enough to defend against rabies, coronavirus, and most other typical diseases for which vaccinations are given. The are accurate assessments of a dog's protection against disease and are used by some to determine if vaccination is necessary. Titres are more expensive than vaccines, typically costing $150-$200 for a dog and take several days to get results.

As a sidebar, England is a rabies-free country, so they do not vaccinate for rabies there. They also have very strict guidelines for allowing dogs into their country--especially from the US, a country known to have rabies in wild animals. If someone wishes to bring a dog from the US, he must first get a rabies shot 3 months prior to the expected date of arrival (whether he is up to date on rabies or not). The vaccinating vet will then send notice to the governmening body of England that he vaccinated the dog to be in compliance. Thirty days prior to the intended date of arrival in England, the same veterinarian who administered this rabies vaccine must do a blood titre and report the results to England. If the dog's titre is not within a specific range, he will still be denied entry to the UK. It is interesting that England insists on the titre but refuses to accept titre results on a dog that may have just been vaccinated 6 months earlier. On occassion a vaccination given at the specific directions of the manufacturer will still not produce appropriate titres.