Booster vaccination or revaccination is not some kind of innovation in the context of coronavirus.
Some vaccinations that you received as a child need a booster.
For example, vaccinations against diphtheria or tetanus.
They need to be repeated every ten years.
As the U.S. Centers for Disease Control and Prevention (CDC) explains, the protection provided by the first dose of vaccination gradually wears off. And although vaccines still work well against severe and fatal cases, recent studies show that the protection is weakening, especially when we are talking about people over 65.
Therefore, a booster dose makes sense. 
The results of one study published in the New England Journal of Medicine showed that COVID-19 cases and severe cases were significantly lower among those who received the booster dose than among those who did not. 
It was primarily about the Pfizer/BioNTech vaccine. 
Other studies show that the booster dose, the third dose of the mRNA vaccine, also helps to develop immunity against the Omicron variant, which can "bypass" protection in vaccinated people.
The booster dose is recommended to be administered within 6 to 9 months from the date of the second COVID-19 vaccination.
Although some countries allow a pause of 5 months after if the main course of vaccination was the Pfizer/BioNTech vaccine. And some countries, such as Australia, recommend a 3-month interval.  However, in Ukraine, most likely, you will be given a booster vaccination in six months, according to the recommendations of the Ministry of Health.
It is also important not to confuse a booster dose with an additional dose.
An additional dose is given to people with a weakened immune system that could not fully respond to the first two doses of the vaccine. And after an additional dose, such a person will receive a booster.