A diode is a polarized component that lets current flow through it in one direction, but restricts current flow in the other direction.
Diodes have two leads, a positive anode and a negative cathode. Typically a diode will have a stripe on one end of its body. The stripe indicates the indicating the cathode lead. The side without the stripe is the anode.
A good way to remember which is which is to think about grades. Getting an A in class is more positive than a C, and for many, getting a C is a negative for their overall GPA. Components that have positive and negative leads are known as having polarity and can only be inserted into a circuit in one direction. If you place an LED in a circuit backwards, it won’t illuminate — that’s because it won’t let current flow through it.