Fuses protect equipment and cabling from excessive current. "Excessive" current is current in excess of the stated rating on whatever device it is you are fusing.

When you place a fuse in circuit, you give the circuit a safe place to overheat, in the case that something is wired incorrectly, used too much, short circuits or otherwise does not act as expected. The fuse is designed to melt when a current higher than its stated limit is passed through it.

Blade fuses in a variety of physical and electrical sizes. The colors denote specific electrical limits (5A, 10A, 25A, etc).

Fuses can be placed in circuit in a few ways. To add a single fuse to a circuit, you can wire in an Inline Fuse Holder. For multiple circuits, a Fuse Box makes more sense, as all your fuses are in one place and can be checked and replaced easily.