The seals are on the valve stems and they keep the oil up in the valve cover from seeping into the cumbustion chamber past the valves.
They are under the valve springs and can be changed without removing the head or valves... a little tricky , but not hard nor expensive.
While doing the job, a tech can feel the play/wear in the valve stem guide and determine if they are also shot. In that case , you are in for a full valve jobs and $$$$.
Also, as a poster mentioned , if they are not too bad and the engine is not using lots of oil , a hotter plug will burn the small amount of oil off and keep the plugs from fouling, but it is just a matter of tme before this no longer works.
Undergapping the plugs a little helps here a bit too, but a bandaid approach, I agreee...