Inserts are the colored plastic circles on a pinball machine that are lit from below via light bulbs. Over time these inserts can become loose or shrink from heat or otherwise sink below the surface of the playfield.
Sunken inserts cause two problems. One – they affect the travel of the ball and two – the ball will start to chip away at the edge of the sunken insert.
I’ve used a couple of methods to raise the inserts to playfield level. One is by dripping on Valathane polyurethane. Letting it dry, then adding more drops, until you get the insert up to level. Then sanding it flat. This takes a lot of time and careful effort and is the preferred way if you are clear coating the playfield.
The second method is purchasing or making a playfield protector. A playfield protector basically puts down a clear sheet of plastic over the inserts and thus eliminates the sunken insert.
A third method is adding mylar circles to the inserts as shown in this video.
I’ve found sources for various sizes of these clear mylar circles on Amazon from the typical 1 inch and 3/4 inch circles as well as small 1/2 inch circles for use on games like Meteor which have some tiny inserts.