The slightly tongue-in-cheek term for the addition of salt and lime to a shot of tequila.
Tequila is tasty, but harsh - the slow burn associated with downing an ounce of the stuff is enough to make one's eyes water and stomach churn, particularly if the tequila isn't the first thing you've decided to drink in an evening (and it never is). The salt and lime bookend the liquor, making it more palatable - the salt makes the tequila sweeter before you drink it and the lime removes some of the punch afterwards. Calling the condiments 'training wheels' is a simultaneous recognition that a. you're not the perfect drinker yet and that b. you're not insane.
Unlike the little wheels that kids put on their bikes, driving this particular one and a half-wheeled mexican monstrosity without help is easier served by taking one wheel off at a time - ditch the salt first, get used to that and then forgo the lime. You'll discover that tequila has a totally different flavor that might be more palatable to you over time.
The one advantage to the wheels apart from the help is that it disguises the flavor of truly awful tequila the way cranberry disguises the taste of well vodka at an open bar - start drinking the stuff for real and you'll suddenly realize that Cuervo just ain't doing it anymore and that your wallet is taking a severe beating. Whether that's worth it or not is up to you.