hi all! just a reminder that current affairs are off limits in this sub: kindly keep all discussion to events & conditions pre-1994 only - thanks!
OP, for responses on current opinion in various countries, consider x-posting this question to /r/AskSocialScience, /r/Politics, or perhaps some of the countries of interest, e.g. /r/AskUK. Meanwhile, there's a section in the FAQ that can get you started
Basically there are two functions that monarchs served, those being head of government, and head of state. The head of government is what it sounds like; they make executive decisions and are involved in the laws that run the country. Head of state is more of a representative role. They do things like meet with other leaders, give speeches to inform or rally the public, or preside over ceremonial events.
Essentially in places like the UK, as a democratic system has taken hold, the monarch has retained the head of state role while giving the head of government role to a parliament.