I'm not a pilot, so I always wonder when I'm flying why does it happen... Is it because of bad weather, or something else?
What is turbulence?
How does it happen?
The main production of turbulence is through mechanical production and buoyant production. Turbulence happens because energy is being put into it via these production methods. Turbulence will decay on its own but will not stop until production ceases.
Both of these methods put energy into turbulence at large scales. For boundary layer flows, this can be on the order of a kilometer or more, typically the height of the boundary layer. These big eddies will have perturbations of their own and these smaller perturbations are the next smaller scale. Energy is cascaded downward until the sizes of the eddies are small enough that viscosity dominates over inertial forces and the energy is then lost from the turbulence as heating. The superpositioning of these eddies at all scales resembles a jumbled bumpy mess, and this is precisely how we experience turbulence.
Air turbulence is very much the same as the currents of a river.
When a parcel or stream of air moves differently than the area around it, you get turbulence as you transition between them.
An example would be if you are outside on a windy day but stand behind a tree to "get out of the wind". If you step out from behind the tree, you will feel a sudden "jolt" as the wind hits you. If it's a particularly strong wind, it could even cause you to lose your balance for a second until you compensate for it. Well, these invisible air currents affect an aircraft in much the same way.
It isn't normally dangerous, and I think of it like driving down a bumpy road. Of course, it's usually even safer in an airplane because even if you hit a big "pothole" you don't have to worry about it throwing you off the road into a tree.
The actual movement of the air can be caused by several things: