Between the 3 lanes there are jungle areas that are inhabited by different kinds of monsters whose two functions in the game is to enable you to gain character levels and get team boosts by killing them. When you kill these monsters you gain EXP for your hero which will help them reach the required level to boost their skills. To get the team boosts will depend on the monsters you kill as some will give you experience while others will give you a certain boost which could either be an attack boost or a health regen.
Fighting in the jungle or 'jungling' as it is often referred to can be very helpful but in order to profit from it you have to know what characters are good for it and what you are aiming for. There is some equipment that is specifically for the jungle so building a character with it will create a character that is ideally suited for the purpose of fighting in that environment. If you do equip your character with the jungle equipment you will need to unlock it which is done by having 'Punish' set as your talent. The ideal characters to choose as a jungle characters are those that are good at dealing out quick damage which makes assassins and warriors the best choice.