A Wheelie is a technique that drivers can perform on Bikes to gain speed in Mario Kart Wii. When players press the corresponding button on their controller, their racer pulls up on the front end to drive solely on the back wheel. Apart from being a pretty cool spectacle, it gains the racer Speed.
A downside to Wheelies though is that the vehicle loses a substantial amount of control; turning even slightly takes quite an effort, because after all, the steering tire is up in the air. For this reason, Wheelies are mainly executed on long straightaways, not turns. Trying to force the vehicle in Wheelie position to turn to much will make the front wheel fall back to the ground, so you can take the curve the standard way.Also, any collision with players while doing a Wheelie will result in a big deceleration and slight spinning out. Normally, bikes would just be bumped around (according to the weight class), with no loss of speed. It's just another factor that makes Wheelies all the more harder and more risky to pull off.
Due to the speed advantage wheelies give, bikes can easily overpower karts in terms of speed in Mario Kart Wii. As well, with experience, performing wheelies will become substantially easier, safer, and easier to apply. It is extremely common for racers to wheelie right out of a Mini-Turbo, abusing the sheer speed advantage. Although karts are statistically faster, the wheelie feature makes that statistical speed advantage almost completely irrelevant. On turns that karts usually power slide through, bikes can abuse multiple mini-turbos, performing a very short-lived wheelie after each one to gain speed and contend with karts that can power slide and get a super mini-turbo.
Due to Snaking becoming much less practical and advantageous in Mario Kart Wii, bikes are superior to karts.
On Time Trials, wheelies are the reason why bikes are the go-to vehicles to get the best times. With almost no risk of getting knocked out of a wheelie, the biker is usually in wheelie formation when not drifting or carefully planning the use of an ultra shortcut.
When bikes returned in Mario Kart 8, Wheelies were removed because they made them way to overpowering to Karts. To counter the loss of the main selling point of bikes, they can now perform Super Mini-Turbos and have a wider Drift radius. Wheelies ARE still possible, but only when using Mushrooms or other boosters... and even then, it's purely an aesthetic animation.