Short answers, pulled from the story.
The Zemsky Sobor was the parliament of the Tsardom of Russia, active during the 16th and 17th centuries. It represented three categories of Russian feudal society: the nobility and high bureaucracy, the Holy Sobor of the Orthodox clergy, and commoner representatives including merchants and townspeople. Assemblies could be summoned by the tsar, the patriarch, or the boyar duma.
The first Zemsky Sobor was held in 1549, convened by Tsar Ivan the Terrible. Ivan used the assembly primarily as a rubber stamp, though it also gave lower nobility and townspeople a channel to address their concerns.
When the Rurik dynasty ended without an heir, the Zemsky Sobor convened in 1598 during the resulting succession crisis and elected Boris Godunov as Tsar of Russia. This gave him formal legitimacy from the assembled estates of the realm.
The Zemsky Sobor of 1613 elected Mikhail Romanov as tsar, ending the Time of Troubles and founding the Romanov dynasty. Assemblies were then held annually in the early years of Mikhail's reign as the new dynasty worked to stabilize its rule.
The last Zemsky Sobors were held in the 1680s, one to abolish the mestnichestvo system and another to ratify the "Eternal Peace" with Poland-Lithuania. By the reign of Peter the Great, the assembly was never summoned again.
On the 23rd of July 1922, White Army general Mikhail Diterikhs convened a Zemsky Sobor of the Amur region in Vladivostok under the Provisional Priamurye Government during the Russian Civil War. The assembly named Grand Duke Nicholas Nikolaevich as Tsar of Russia, but the plan collapsed within two months when Bolshevik forces took the region.