Names and dates are the skeleton of a family tree. Sources are what make it worth trusting. The Sources page in GED-VIEW shows all of that at once instead of one citation at a time on each person.
Three words
A repository is where a record is kept: a county archive, a church, a website, a shoebox.
A source is a document or a collection: the 1900 United States census, a parish register, a book. In some files a source is one big collection cited hundreds of times; in others each individual record is its own source. Both are legal and both are common.
A citation is the link between a source and a fact: "this birth comes from page 14, line 3 of this census." One source can have hundreds of citations. That is what the number next to each source is counting.
The layout
The main list is every source in the file, searchable and sortable. Search covers the title, author, publication details, and any transcribed text stored with the source. That last part matters: many files carry the actual wording of a record inside the source, and searching that text is often the fastest way to find a specific document.
Click a source for its details: author, title, publication, which repositories hold it and under what call number, attached scans or documents, and the full list of citations pointing to people and events. Click a citation to go to the person.
The repository filter narrows the list to sources held at one place. Some files have detailed repository records; some have none. Ancestry exports in particular tend to arrive with almost no repositories, so an empty filter is the file, not the app. Sources without a repository sit under Other sources.
Sources can be pinned to the top of the list and hidden from it, which helps when a file has hundreds and only a few are the ones you keep coming back to.
Two fields worth knowing
PAGE is where in the source. Free text, filled in differently by every program: a page number, a line number, a URL, a sentence. It is what tells two citations of the same census apart.
QUAY is a rating from 0 to 3 that whoever entered the record gave to its reliability. 3 means direct evidence from a primary source; 0 means unreliable or estimated. Most people never set it, so most citations show "not rated". When it is set, GED-VIEW shows it, and the list can be sorted by it.
Scans and documents
If the GEDCOM references image files and a media folder is connected, any scan attached to a source or citation appears next to it, with a preview. This is where the media folder pays off most.
If a source has an attached file that did not match anything in the folder, GED-VIEW shows the reference greyed out with the file name it was looking for.
Vendor extras
Some desktop programs store extra structure inside a source that the standard has no place for. RootsMagic exports its source templates as a set of named fields; GED-VIEW shows those as a small labelled table on the source page rather than discarding them. Ancestry exports carry a reference number that identifies which Ancestry record collection a citation came from; it cannot be clicked through, but GED-VIEW shows the collection number so you know where a fact originated.
The report that matters most
The most useful thing the Sources page enables is on the Source coverage page, under Research: a list of every person in the file with no sources at all. That list is a research plan. There is a separate guide on working through it.
Next
Open the file, go to Sources, and sort by citation count. The sources at the top are the ones the tree stands on.