Lets start simply.
No question is stupid. if you need to know something or don't quite understand then your question has to be asked.
PE detail is a way of adding extra detail to a model subject. Some top end kits come with quite a lot of this stuff as part of the build and it is a way of providing detail that can't be done in styrene and molds.
For instance, my current build is Amusing Hobby's 1/35 E-100 Waffentrager and the only bit of PE in the kit is this:
This is very simple PE to work with. All it does is add a wire mesh to the top of the air intakes and cooling grills on the engine compartment.
This however is different. Rye Field Model chose do do parts of the lower hull internal frame of their Panther kits in Photo Etch. You can see the internal frame (the thin stuff) in place.
Takom did those parts in their panther kit with interior in plastic I believe. It is a way of doing detail in scale thickness that would not really be possible in the molding process.
Sometimes you have to use the kit stuff as part of the build if it is included in the box, but the majority of PE is in aftermarket sets that are there to either upgrade kit parts with much better and finer detail or replace the kit parts for the same effect.
THis picture is of Eduards release of their cockpit update set for HK Models' 1/32 Lancaster.
THis set will replace all the panels in the kit with detailed etched and coloured pieces. Just scrape off the molded detail on the parts and stick those on in their places. there are also some parts that will replace or upgrade some areas, but this set mainly replaces the instrument and radio faces etc.
These things replace the task of painting instrument panels and applying decals and dials that can take quite a lot of effort and time. When applied correctly, these replacement PE sets look fabulous!
Of course, you don't have to use them if you don't want to. The main aim is to offer more detail or to provide correct scale thickness
James