Q:

What does it mean when the borderlands symbol flashes on my screen?

Occasionally when playing, I see the symbol below appear. It usually flashes for a moment and then disappears.

What does it mean?

A:

It's the autosave icon; it indicates your game has been saved. There's a screen that explains that either when the game starts up or as one of the Loading Screen tips.

A:

I don't know about Borderlands 2 in specific as I don't have it, but in a great many other games that indicates an autosave going on in the background. 
Normally you're given a warning when you start the game saying "don't turn off when this happens, it's bad".

