is short for "In my opinion". There are a lot of acronyms that will show up highlighted and underlined like in the posts. Move your cursor over the word and the definition will appear in a box. You'll get the lingo as you get more into the hobby and more involved with the site.
What I mean by overskimming is just an opinion I have. I really do feel like you can have your water too clean. I think the ideal conditions mimic that of natural waters as closely as possible. Basically, I think you need a certain level of dissolved organics, trace elements, and protiens in the water that overskimming and over filtering can remove. Again this is my opinion only, as I said, if this used skimmer is in good working order it is well worth it.