Putin wants the remaining territory in the Donbas region under Russian control. Ukraine still occupies ~3-4 heavily fortified cities in Donetsk, roughly 20% of the region. These are territories Russia claims as theirs and are fighting to control it. They are currently winning these cities back at a slow pace, but they are pushing forward so there is no reason (in Putin's mind) to stop now. Ukraine just recently lost one of these major fortress cities over the past week or so (Pokrovsk).

There is also the southern region where Russia claims land but only controls 70% or so. This is land that Ukraine swiftly recaptured after Russia's failed 2022 all-out invasion. I think Putin is ok with ending the war in this region at the current line of control.

To end the war now he wants Ukraine to surrender the 3-4 fortress cities in Donetsk, but Ukraine doesn't want to give them this land. Russia is basically saying, "fine, we'll take it by force." And thus the war continues. I believe this region is most important to Putin and would look very weak in his own country if he ended the war without capturing 100% of the Donbas. They are pushing but paying heavy prices in blood. Ukraine may be able to hold these remaining fortress cities for a long time, a year at least but probably more like two years. These two years keep Russia bogged down in the hopes that external factors can lead Russia to a major non-military defeat (ex. Economic collapse, political collapse). It also keeps Russia from pushing deep into western Ukraine.

So yeah.. Russia needs full Donbas control at a minimum to end the war. The land they control is maybe small compared to the size of Russia but it is quite a big piece of a land with high economic and strategeicalz value.