Basically Feeling Musical, or feeling like singing/playing an instument, etc.
"I feel songative today!"
"I'm feeling songative, I think I'll play my guitar"
It basically only means understand. Its a way to make yourself sound like a 6 year old.
"Dude: Hey, sorry I couldn't go for a walk with you today. Mom wanted me to go to town."
"Dudette: Yeah its all right, I underconstumble."
"Dude: what are you, six?"