If you get a virus in one of your cells, it copies its DNA into your DNA in order to make more copies of itself.
If that happens in a blood cell, then that one cell has the DNA insert.
If EVERY cell in your body has the DNA insert, it's part of your DNA.
That ONLY happens when a virus gets into a sperm or egg which goes on to produce a living being.
From then on, anyone born to that lineage will also have the same retroviral insert in their DNA.
So, if this happened to your grandfather's sperm, it caused you dad to have it. Your dad passes it on to you, you pass it on to your children.
That's a retroviral insert.
Any given virus COULD be an insert and COULD insert into any part of the gene sequence.
So, for you and I to have the same insert in the same place on our DNA, there are only two possibilities.
A) Someone a long way back had the virus and we both descend from that person.
B) The EXACT same virus infected two different people, and randomly inserted itself into the EXACT same spot on the INCREDIBLY LONG DNA sequence (3 BILLION pairs = 3 BILLION possible places to insert itself), and then that cell HAPPENED to be a sperm and HAPPENED to result in a kid.
The odds against B are very long, but not impossible.
However, you and I don't just have ONE retrovirus in common, we have many (let's say 30 for the sake of the discussion).
So, either you and I descend from one individual who had the 30 virus inserts -or- B happened 30x in a row without ever having a single mistaken location or additional virus.
Odds against that? Too long to have ever occurred in the entire history of the universe.
So, the fact that ALL humans have the same retroviral DNA inserts means that ALL humans share a common ancestor who had those same inserts.
Get it? Good.
Now, understand this. Of the 30 retroviral inserts we all share, chimps ALSO have 15 of them.
That means that at some point, a long time ago, when there were only 15 retroviral inserts, an individual existed who was both the forefather of all humans and the forefather of all chimps.
That's common descent.
if two people have the same retroviral insert in the same place in their DNA, they likely share a common ancestor.
And we've concluded that if two people have more than one of the same retroviral insert in the same place in their DNA, the odds against them not sharing a common ancestor go up exponentially with each additional insert.
Onto the next point:
Looking at insertion points for one retrovirus (HERV-K) here's what we find:
# of retroviral insertion points in common between humans and...
New World Monkeys: 2
Old World Monkeys: 4
Gorillas & Chimps: 11
Other Humans: 14
So what does that tell us?
It tells us that we share a common ancestor with all of these.
It tells us the order in which the various groups split off.
The least # of viruses in common split first. The most # split last.
The odds against humans and chimps both aquiring 11 common inserts? 1:1 followed by 132 zeros.
That is why DNA researchers consider ERVs to be slam-the-door positive evidence for evolution.