Booster doses of a vaccine are very common in the field of vaccines. Boosting means getting another dose of vaccine at a different time after the initial dose(s) of a vaccine, which scientists call the “priming” stage. It’s like wall paint: priming coat, maybe a second priming coat, followed by another boosting coat. For vaccines, the boost is weeks, months, or years later, rather than a day or two for wall paint. Like a second coat of paint, the boosting improves upon the priming.
Most vaccines we get have boosting components. For hepatitis B, babies get the vaccine at 1, 2 and 6 months. The 6-month vaccine is the boost. For the measles vaccine, children get it at about 12 or 15 months and then the boost 5 years later when they enter elementary school. For tetanus, you get a booster every 10 or so years.
The additional doses given to immunocompromised individuals are to improve their immune response to the initial vaccine series. They often do not build enough, or any, protection against the virus from the original vaccination.