Translation:I have been thinking about what you said to me.
"Ho pensato" presents the thought as concluded, while "sono stato a pensare" refers to a prolonged thinking. You could get the same effect with the imperfect "stavo pensando", although that's perhaps closer to "I was thinking".
Thanks! That makes sense, although one of the given translations is 'I thought about ...' which would more closely mesh with the ho pensato than the sono stato translation, I think.