A middle is an arbitrage opportunity where if the event has a certain outcome, you can win both bets, or get one of the bets returned. This occurs when the plus side is bigger than the minus side, in case of handicap bets. In the case of over and under bets it happens when the over is smaller than the under. To simplest way to explain middles is by showing an example.
If you bet at Unibet on over 2,5 goals and under 3,5 goals and follow the standard surebet mathematics, you have an guaranteed profit of 3.5 percent. However, if the match ends with exactly 3 goals scored you will get both sides of the bet paid out, giving you a profit of 107 percent.
The over/under middles as shown in the example do not show up often. However, middles in for example total points scored in a basketball match are far more frequent. Furthermore, the example shows as a surebet middle. If you want more information about middles in general I would highly recommend reading the strategic use of non surebet middles provided by surebet.com.
Polish middles, also known as inverted middles are more risky and less popular under surebet and middle players. However, they could turn out to be a smart way to win money in the long term. In a Polish middle you bet on an outcome not to happen. So for example you bet on under 2,5 goals and over 3,5 goals. If 3 goals are scored you lose both sides of the bet. However, your profit will be greater than normal surebets, but the risk is also greater. Do not bet on Polish middles if you do not know what you are doing. A mathematical example of Polish middles is provided by Oddstorm.com.