1N5408 is a diode of 1N540x series. It is a power diode. It can be considered as an electronic check valve that allows current to flow in one direction only. It flows from the anode terminal to the cathode terminal. It carries low resistance in one direction and very high resistance in another direction. There are two terminals on the diode which name are anode and cathode. They are attached to the p-n junction. The cathode terminal is identified by the grey colored band.
For 1N5408 Diode, it comes with a peak reverse current of 10uA while forward voltage drop is equal to 1A. The maximum current carrying capacity is 3A it withstand peaks up to 200A. Hence we can use this in circuits that are designed for less than 3A. The reverse current is 10uA which is negligible. These diodes have a slow recovery time so in modern electronics it is being replaced by other advanced diodes.
It is manufactured in DO201 package that is a slightly bigger package from DO-41 but can be used in replacement of many DO-41 diodes if slightly bigger space is available.
It is also an ideal diode if you are looking for a little bit more ampere handling capabilities as compare to DO-41 diodes for example 1N4007, 1N4148 etc. In most applications you can replace these diodes with 1N5408 to get more forward current.