Short answers, pulled from the story.
The Pact of Steel was signed on the 22nd of May 1939 by German Foreign Minister Joachim von Ribbentrop and Italian Foreign Minister Galeazzo Ciano. It was a military and political alliance between Germany and Italy.
Japan declined because of a strategic disagreement over the pact's focus. Japan wanted the alliance directed against the Soviet Union, while Italy and Germany wanted it aimed at the British Empire and France. Unable to reconcile this difference, the pact was signed without Japan.
The secret supplementary protocol, not made public at the time of signing, had two parts. The first urged both countries to accelerate their joint military and economic cooperation. The second committed them to coordinate on press, news, and propaganda to promote the Rome-Berlin Axis, with each country required to post specialists in the other's capital.
The original proposed name was the Pact of Blood, but Mussolini was told it would likely be poorly received in Italy. He proposed the Pact of Steel instead, and that name was ultimately chosen.
The pact was designed to last ten years under Article VII, but it effectively ended in September 1943. After the Allied invasion of Sicily and Mussolini's overthrow by 19 members of the Gran Consiglio, the new Italian government under Field Marshal Pietro Badoglio signed an armistice with the Allies, making Italy a non-belligerent.
Italy was not yet militarily prepared to meet its obligations under the Pact of Steel when Germany invaded Poland on the 1st of September 1939. The pact had been based on the assumption that war would not occur for at least three years. Italy did not enter the war until June 1940.