There was no encryption.

The messages were broadcast over the radio as personal messages for people in France, for example someone who had managed to flee the country before invasion sending a message to relatives.

They were in simple code - a famous one just before D-Day was "The dice are on the mat" (at least thats the rough translation).

The messages were used to arrange drops of weapons, secret agents or to sabotage infrastructure (railways in particular).

The Germans may well have suspected that these messages were for allied forces, so the allies also made broadcasts with lots of fake messages, for example "John has a long mustache" - it meant nothing, but the Germans didn't know that. Their response was to ban all radios in occupied France - there were very serious repercussions if you were found in possession of a radio receiver. Possession of a radio transmitter (as used by the resistance and secret agents) was a death penalty.

A few more examples here: http://www.struthof.fr/en/testimony-of-the-resistancedeportation-and-remembrance/testimonials-about-resistance-deportation-and-memory/the-various-forms-of-action/the-bbc-and-personal-messages/