A tilt is when you tilt your control stick in a certain direction only a tiny bit, not all the way like for a smash.
Forward tilt is fairly easy to do, just hold the stick in a horizontal direction, and press A. If you need to at first, you can start walking (not dashing) first, but eventually you should be able to do it from a standstill.
Down tilt is the easiest to do. Just crouch and press A, no problem.
Up tilt, the one you saw, is probably the hardest. You press up (very slightly) on the control stick, and press A. Alot of time people have trouble with either jumping or up smashing when they try and up tilt. This is because they are pushing the control stick up too far.
Marth's tilts really add a whole new level of depth to his game. Down tilt can be use for edgeguarding, forward tilt is a good spacing tool, and up tilt is one of his greatest moves. Learn them well, especially up tilt, which is (in my opinion) Marth's best move, since it launches them up into the air, and sets them up for tons of aerial comboing. ^_^
