Speedloaders are an often overlooked important part of your kit. No one wants to stand there and load 120 bbs by hand! Speedloaders come in a number of styles. The three most popular sizes are Tube (essentially a plastic tube where you fill it with BBs and then use another plastic rod to drive them down the tube and into the magazine), Thumb driven (These are normally held in one hand and can resemble a pistol magazine or sometimes and M4 magazine depending on the capacity. This type of speed loaded has a plunger that is pushed down with the user's thumb to push the BBs into the magazine) and winding (These are normally larger and hold a lot more BBs. They use a crank on the side of the speedloader to turn a wheel and push bbs down into the magazine).
This is also where you can find those other airsoft nicknacks that you never knew you needed.