They're a 3rd party company that makes replacement displays for machines. So say you wanted to replace an old alphanumeric plasma display, they'd sell you a new LED one that's a drop in replacement.

Mylar is the clear sticker that you can put over the playfield to protect it. When it gets warped etc it can bubble up, so you can see air pockets in it.

Mylar is a clear plastic film that's used to protect parts of the playfield , if there's bubbling in it that means that parts of it are trying to come off.... Can be a pain to remove it too

More specifically, it means that air has gotten under the mylar because the adhesive has weakened. Usually mylar in this condition is much easire to remove than mylar that is fully flat on the playfield. Bubbling mylar not only looks bad, it can affect ball travel. It should be either removed and replaced, or removed and left off - which is what I prefer. There are two prominent methods for removing the mylar and many threads about them: freezing or heating to release the mylar sheet is the first step, then rmoving the glue which is a process in itself which can be even more difficult to complete.

Read up the threads on these processes, if you can possibly practice on an old used playfield that would be even better. Personally I have had excellent results with the heat method using a heat gun, with no damage to the playfield at all. The most important thing to engrave in your mind with either process is TAKE YOUR TIME and do not rush. Good luck!