What does the engine code P1870 mean, please. Transmission.

Thank you both. I could not find any specific information for this code even on the internet. Yes, I feel the tranny-car stuttering as I drive it at some times. It seems better if I drive it in 1, 2, and 3, starting in 1 and shifting it up through 2nd and 3rd. Stutters in drive on pickup sometimes and seems to be getting worse. The P1870 code does not come on very often, just at higher speeds on the highway mostly.