Diodes 1N4148

A diode has low (ideally zero) resistance to current flow in one direction referred to as forward bias, and high (ideally infinite) resistance in the other direction referred to as reverse bias.

The end with the bar is the negative or cathode side of the diode. Diode Diode symbol
LED will turn on in a forward bias connection.
Diode forward bias
LED will not turn on in a reverse bias connection.
Diode reverse bias