Apical Dominance
Apical Dominance is when the main tip of a plant keeps side branches from growing out.
Think of it this way
A big bowl holds a stack of blocks. One block sits on top and presses down on all the blocks below it. The blocks below cannot spread out or rise up as long as the top block holds them down. Remove the top block, and the lower blocks push out to the sides.
Explaining apical dominance by grade level
A plant grows one tall top shoot. That top shoot sends a signal down the stem. The signal tells the side buds to stay small. Cut the top off, and the side buds grow.