Only one thing directly happens when an @21 instruction executes: the A register is set to 21.
The value in the A register has two side-effects.
One side-effect is that when an instruction uses the M "register" it accesses RAM using the address in the A register. For example @21 followed by D=M will read RAM into the D register. This side-effect is what is being referred to as "the selected RAM register".
The other side-effect is that any instruction that jumps will use the address in the A register as the destination address for the jump. For example @21 followed by 0;JMP will cause the next instruction to be read from ROM.