What was the importance of the Zimmerman plot in World War 1?

The Zimmerman telegram was sent to Mexico, but intercepted by Great Britain who then decoded it and told the US about it. The Zimmerman telegram basically said "if you join us and help defeat the US, then we will help you get your land back that you lost in the Mexican War"

The Zimmerman telegram was the finial act that made the US join the Allied Powers on WW1

I hope this helps!