Generally a "high commitment" play is one that's also one that's higher in risk.
When player's are playing low commitment, they're mostly using safer moves and trying not to leave any big openings in their gameplans. Generally moves that are low in start up and/or endlag are considered as low commitment, as well-spaced moves with a lot of range. High commitment moves will generally be ones that leave you open, such as smash attacks or moves that are easily reacted to.
For Mario, a some examples of low commitment moves would be his Nair and Bair, since they have good frame data and are generally safe on shield. Some high commitment options he might use are F-Smash or Forward Air since they come out slow and a wiffed or blocked one will often leave an opening.
Generally skilled players only go for moves like these when they're confident they'll land them or won't be punished. So generally it's a good idea to start a game playing "low commitment" in neutral and then start using higher commitment moves more when you've figured out some of your opponent's weaknesses or habits.
