Simple terms as you're obviously in training (this isn't really a "tech question" either)
Sequence the flight plan means make the next waypoint in the fmgs the next waypoint you are expecting to fly to after leaving radar vectors or heading. For example, during an SID, ACT give you radar vectors and take you further away than the 6ish miles required to automatically sequence the flight plan and you end up 30 miles downroute but the next waypoint in the box is the second on the SID. This results in incorrect timings and fuel predictions. It will clear itself when you hit DIR TO but in the meantime delete the points you have already flown past.
Extending the centre line is the same idea. Once ATC take you off the STAR onto radar vectors, you can extend the centre line by entering a DIR TO the centre fix with the radial out entered as the inbound course. Confused? Practice. You'll get shown it during line training. It gives you a more accurate distance to go figure.