20mm long x 5mm diameter, 4A time delay (slow blow) fuse.
Rated for use up to 250 Volts.
A fuse is a sacrificial device which provides overcurrent protection, of either the load or source circuit. It's essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overloading, mismatched loads or device failure are the prime reasons for excessive current. The fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented.
Slow blow fuses are designed to allow harmless short term higher currents but still clear on a sustained overload. These types of fuse are used on equipment such as motors, which can draw larger than normal currents for up to several seconds while coming up to speed.
When replacing a fuse, always use the correct size / type of fuse and make sure the circuit is isolated.