Just like the vaccines which are being used, the booster shots also contain either inactive/ altered/ weakened/mRNA fragments which train the immune system. The primary reason why booster shots are being introduced right now is to target the many variants of concern that have been clinically found to lower the efficacy of the COVID vaccines we have right now.
Additionally, booster shots can also be altered or tweaked to suit specific needs or offer preventive protection against variants of interest, or concern.
A booster shot, once injected prompts into work memory cells in the body, which ‘remember’ the infection and specific mutations that are concerning. So, the injection of shots can be particularly useful against both the original variant (Alpha variant of coronavirus) and additional mutations (such as Beta, Delta, Gamma, or Delta plus). The additional dose can thus ‘boost’ immune defence against the original concerning variant, prompt more antibodies, and if exposed to the different variant, dole necessary immune defences to play.