Learn Polish: Nowe kierownictwo zmieniło zasady. - The new leadership changed the rules.

It derives from the noun kierownik (manager) or the verb kierować (to direct/manage). In this case, the -stwo suffix creates a collective noun. Instead of an abstract idea, it refers to the whole group of people performing the action, meaning the management or leadership team.

Does kierownictwo refer to the people in charge or the abstract quality of leadership?

It almost exclusively refers to the group of people (the management or administration). If you want to talk about the abstract quality of being a good leader, you would use przywództwo or zdolności przywódcze (leadership skills) instead.

Even though kierownictwo refers to a group of people, why is the verb zmieniło in the singular neuter form?

Because grammatically, kierownictwo is a single neuter noun, as indicated by the -o ending (which is standard for all nouns ending in -stwo). In Polish, collective nouns take singular verbs that match their grammatical gender, regardless of how many people are actually in the group.

What case is zasady here, and why doesn't it have a special ending?