Decipher means translate from code, or more generally, figure out. If you can’t decipher your teacher's writing, it means you can't read it. You might feel like you're being asked to decipher a code.
During World War II, the Nazis used a machine called the Enigma to encode secret messages. An operator would type the message with a keyboard, which would activate a set of rotors, giving each letter a different value. The code was difficult for the Allies to crack, but eventually the code was deciphered by the British, hastening the end of the war.