After entering a flightplan and flying the route I obtain the message ( seeattached picture ) which I do not understand and especially what kind of actionsare required.What means the word DECEL and what is required ? Thanks for your help Hubert Werni

DECEL is the deceleration point where you are going to start slowing down to get back on the decent path. CHECK ALTITUDES (I think) means that the altitude in the FMS does not agree with the altitude set on the FCP.