Bonus Action

Various Character Class features, spells, and other abilities let you take an additional action on your turn called a bonus action. The Cunning Combatant feature, for example, allows a Rogue to take a bonus action.
You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action. You otherwise don't have a bonus action to take.

Bonus Actions in Combat

Most bonus actions are designed to be used in combat.
You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available.
You choose when to take a bonus action during your turn, unless the bonus action's timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus action.

Bonus Actions outside of Combat

Sometimes you might want to take a bonus action outside of combat, such as while you are adventuring.
Outside of combat, when you have the ability to take an action as a bonus action, functionally it means that you can take the bonus action at the same time that you are pursuing another action, or that you can take that action as an immediate action instead of a committed action (see Tracking Time. 
For example, a Jack has the ability to speak and draw a crowd's attention as a bonus action. That means the Jack could also cast a spell while speaking to the crowd -- a trick that another character might find difficult.
Another example: a Spy can use a bonus action to try to influence a creature. Normally, according to the rules that would take 10 minutes, which is a committed action while adventuring. A spy can do so as an immediate action, and attempting to influence the target will not advance time.