Yeah, this is the war Ukraine has been fighting all along; they're just able to do it more.
It pushes back supply lines. Ammo takes longer to get to the people who need it. If something goes wrong for the Russians on the front, it takes longer to get reinforcements. When Ukraine mounts an offensive, it weakens Russian defense. It also reduces how far into Ukraine Russia can strike with some types of artillery, because they have to position the guns farther back. Staging is harder; Russia has to sneak together assault groups farther, giving Ukraine more opportunity to notice and prepare.
Ukraine is making it harder for Russians to keep troops supplied, forcing them to stretch their supply lines, which also make them easier to attack and disrupt. The farther back they can push staging areas, the harder and slower it is for Russia to mount assaults.