To encourage Marty to get it, like get it girl!
When Marty Friedman is playing his guitar on stage and shredding.
"Oh wow! Get it Marty!"
"Thanks!"
Get it Marty, meaning to encourage somebody, cheer them on for doing anything really. They're normally referring to the former guitarist of Megadeth, Marty Friedman.
It started off on Pinterest when someone commented it, and then others do it. So it's mainly just an inside joke for the Marty Friedman/Megadeth fans. Whenever they see a picture of Marty they say "Get it Marty!"
"Wow! Look at Marty playing the guitar, he sure can shred!"
"Get it Marty!"