The term tipper can be used as a general term of reference for hitting with the tip of any attack, but is usually used in terms of the Home-Run Contest and Marth and Roy's attacks.
In Home-Run Contest
A tipper occurs in Home-Run Contest when one hits Sandbag with the tip of the bat during a bat smash. It deals 20% damage to Sandbag and sends it farther than other bat swings (a close hit or midhit). This is the usual method of finishing Sandbag, unless the character has an attack that is stronger such as a Warlock Punch.
An aerial tipper is a tipper in which one hits Sandbag while it is still in the air. It sends Sandbag farther than a normal tipper.
With Marth and Roy
Outside of Home-Run Contest, "tipper" is typically used in reference to Marth and Roy's attacks. Marth's tippers are stronger than his center hits, as it is the sweet spot on his sword; Roy's tippers are weaker because his sweet spot is in the center of his blade.
Other uses of "tipper"
The term "tipper" can also be used with Bowser's forward smash and a few other attacks.