Germany offering Mexico an alliance for the case that the USA enters the First World War. Offering the territory that Mexico lost in the Mexican-American war. This was for the case that the unrestricted U-Boat warfare would lead to the USA entering the conflict. A land war would keep American troops out of Europe for some time, giving the Germans an advantage.

The message was decrypted by the Brits who held it back for some time, cause they didn't want to reveal the fact that they had broken the German codes. But eventually they were able to publish the message without revealing that secret.

It's really not very long.

You can read the full text here.

If you really want a summary, it said that the Germans planned to resume unrestricted submarine warfare, that a Mexican alliance was wanted in case the US were to enter the war (so Mexico would declare war on the United States to distract them, and they would regain Texas, New Mexico, and Arizona, plus financial support from Germany), that the English would fall shortly thanks to this submarine warfare, and that the Mexican president should talk to Japan to make sure they were in on the plan if he liked it.

It's really straightforward, though.