The following is what he told me, I think that's really nice clear explaination, so I didn't decide to change his words.
" Well intro basically means the cool part in front of a video or when you go on to someones channel, its the cool thing you see right before videos that say the persons name, and something like there URL or "presents". A template is something you can get that allows you to edit it, such as a background, lets say you want to change the colors, but you cant casue you have a JPEG file. A template basically allows you to put some finishing touches after i make most of it (: hopefully that solves some of your questions (: "
And by me, BG is short for background, we can simply use a whole color as a background, but most will not think it's enough, because we want to make our channel the charming figure, so find ways to get a cool background. We can search in google with the content "youtube template", then will show many youtube template results, choose one to edit in your photoshop, then after editing, you can even save this as your template, cauze you may need it again when you want to reedit your youtube BG.
I do appreciate his nice explaination :) , and as time goes, I found that most youtubers are very kind to help you. So, if you have a question, don't hesitate to ask them, I think they will be happy to help if they can and have the time. Youtube is a big platform, where are all kinds of super people, if you want to get an answer, you will, like the internet.
Thanks for reading, best wishes ^_^