Short answers, pulled from the story.
The Locarno Treaties were seven post-World War I agreements negotiated among Germany, France, Great Britain, Belgium, Italy, Poland and Czechoslovakia in October 1925. The formal signing took place in London on the 1st of December 1925. They collectively guaranteed the western borders of Germany and established arbitration procedures for settling disputes peacefully.
Foreign Minister Gustav Stresemann calculated that appearing cooperative would win Allied goodwill and gradually restore Germany's diplomatic freedom. He deliberately accepted the western borders with France and Belgium while refusing any guarantee of Germany's eastern borders with Poland, leaving open the possibility of future territorial revision in the east.
The spirit of Locarno referred to the improved political atmosphere in western Europe between 1925 and 1930, built on expectations of continued peaceful settlement of disputes. Historian Sally Marks described it in her 1976 book The Illusion of Peace as a fragile foundation, and Hans Mommsen noted that the French premier's declaration of a new era of trust never became reality.
Poland was unhappy because the treaties guaranteed Germany's western borders with France and Belgium but imposed no equivalent guarantee on Germany's eastern borders with Poland. Polish military attache Józef Beck said Germany was officially asked to attack the east in return for peace in the west, and the treaties contributed to the fall of the Grabski cabinet on the 14th of November 1925.
Austen Chamberlain received the Nobel Peace Prize in 1925, and Aristide Briand and Gustav Stresemann shared the prize in 1926, as the lead negotiators of the Locarno Treaties.
The Locarno Treaties effectively ended on the 7th of March 1936 when Nazi Germany sent troops into the demilitarized Rhineland. The other signatories offered only verbal condemnation; Italy had already promised not to act, and France lacked British support for a military response.