This song is about Rhett's sister-in-law, Ashley. He talks about it in a Rhett and Link podcast called ear biscuits.
I think it was in the episode where they talk to Tyler Oakley or Shane Dawson. Apparently Scott Avett and Ashley were together for a while. Rhett emailed them and asked if they could meet but they never responded. I recommend checking out that podcast. It's pretty entertaining. I also recommend good mythical morning if you haven't already seem that on YouTube.