What is a rectifier diodes?

It's an electronic component used to turn AC into DC. But it'won't be pure DC w/o some further processing. It is basically a semiconductor meaning will pass current one way only. Electrons will flow from positive to negative. Therefore Direct Current or DC as opposed to AC alternating current meaning positive and negative current flow. there is no such a thing as pure DC by adding a capacitor it can approach pure DC but mathematically speaking it is impossible to achieve.