The AGBI on Ancestry is basically an index of an index. It's referencing a big series of books that indexes tons of sources on early Americans. I don't know why Ancestry's index doesn't include all the data included in the book's index, but it doesn't. So to find the original source, you first have to look up the AGBI book index. You can find the books in a number of places online, I usually use the one at FamilySearch because it's free and accessible from home (it's not a restricted collection) - scroll all the way down (passed the listings that say off-site storage).
Ancestry's index should include a volume and page number, although weirdly the books don't include page numbers, that's okay, because they're in alphabetical order. So simply open the volume you're looking for, and then find the name you're researching in alphabetical order. There will likely be several entries for the name you're looking for, but you can usually tell which one you need from the location and/or time period included in Ancestry's index. Even so, the AGBI books can be seemingly as vague as Ancestry's index is, and sometimes it takes some understanding and/or Googling of what it's referencing.
For example, "Pa. Archives" is not a reference to the Pennsylvania State Archives, it's a reference to another series of books that includes primary records from early Pennsylvania called the Pennsylvania Archives - there should be a series number, a volume number, and a page number. The Pennsylvania Archives are also available online at various sites, Google Books, Archive.org, Ancestry, FamilySearch, Fold3, etc.
Another example is a source just called "Transcript" - this is a reference to the Boston Evening Transcript, a newspaper that ran a genealogy column from 1906-1941, including details on ancestors not exclusive to Boston or Massachusetts. Obviously, it's very much a secondary source, so I'd be careful with it, but it's available from Newspapers.com covering the years 1848-1914, and at American Ancestors covering 1911-1941 (select the Boston Evening Transcript from "Database").
You'll also see references to Revolutionary War Rolls and Pensions, those are fairly self explanatory. There's also states with "Heads Fams" which is referring to the names of the Heads of Families listed on the 1790 census. Since the 1790 census is already widely available online and probably already attached to your tree when appropriate, this isn't a very useful citation anymore. There's lots of other sources included in the AGBI, but usually they are self explanatory, or you can find out what they mean with a little bit of Googling.
It's really important that you find the original record the AGBI is referencing because the index is so vague, there's really no way to know from it whether it's for the right person you're researching or not. You may often find that once you look up the original record, it's not actually a reference to the person you're researching after all. Probably, researchers on Ancestry just attached it to their tree because the name and perhaps location and/or time period fit, without looking into it further probably because they simply don't know how to find the original. But particularly with common names, you can't assume that means it's for the correct individual, and now you don't have to be one of those people.