P1391? It's pretty much as described, wesley... it means the engine controller (PCM) has seen an instance where cam or crankshaft position signal has been lost momentarily but it can't decide which one it is. Cmp and ckp are shorthand for camshaft and crankshaft position sensors.
It may be that both disappear momentarily or only one does, but is too fleeting to be properly diagnosed by the on-board diagnostics.
I might be able to shed a bit more light on your problem if you'd send me information on the make, model, year and engine of your vehicle. If you're not sure about any of these items, sending me the last 8 digits of the VIN will allow me to access the build information in great detail.
Also let me know if there are any driveability problems associated with the P1391... stalling, bucking, surging, misfire... or just a MIL illumination.