Trick usually means using a feather at the start/finish line and having Lakitu pick you up and finish the lap for you. Open means to cut a huge portion of the track and have you cross the finish line on your own without the help of Lakitu.
Basically this TAS video exploits all of the tricks possible in SMK:
MC1- mushroom longboosts (OPEN)
DP1- lap skips by bumping into the wall (TRICK)
GV1- start/finish Lakitu lap skips (TRICK)
BC1- all of the feather shortcuts are considered OPEN besides the very last lap, that's considered TRICK
MC2- all of the mushroom longboosts are considered OPEN, but landing on the wall and skipping the laps are considered TRICKS
And so on. Essentially whenever you blatantly skip a lap, those are tricks. Cutting portions of tracks and crossing the finish line with skill will always be open.
I hope that makes it more clear for you!