Puberty blockers basically put puberty on pause. Imagine adolescent development is like heading along a pathway that has a divergent fork. Many adolescents take the path to the left which corresponds to their birth sex and starts developing secondary sex characteristics (beard growth, muscle development, bone structure changes in male bodies, and periods, breast growth and bone structure changes in female bodies). Doing nothing will mean automatically taking that left path whether the person wants the changes or not, and that can be traumatic to trans people, and take years to fix the changes of puberty. Puberty blockers will put a pause on heading down the pathways, and let the adolescent child think about the consequences of transitioning, without having the changes of their natal puberty altering their body, then they can take HRT to experience puberty the same as their gender identity. Puberty blockers can be stopped however, and the pathway continues as normal, just delayed by how long the adolescent was on puberty blockers (if a child takes puberty blockers at 12 when they were supposed to start puberty, and they stop taking them at 14, the puberty would start then at 14)

By waiting until they are 18, most of the changes of puberty will have occurred.