The Second Amendment to the Constitution protects a U.S. citizen’s right to possess and hold firearms. It was ratified on December 15, 1791 as part of the Bill of Rights, at a time when the newly formed states “[were] concerned that Congress might try to disarm the state militias. The state militias were then the chief military force of the nation.” (https://www.minnpost.com/eric-black-ink/2019/05/second-amendments-origins-laid-out-in-compelling-fashion-by-igor-volsky/)
“To bear arms” simply means to carry them, so granting US citizens the right to bear arms means allowing them to walk around with them. For this reason, many US states today allow their residents to carry weapons in public without fear of arrest. In many states, gun owners do not even have to register their arms. They can buy them legally and carry them in public without any sort of government oversight.
In 1791, militias were groups of men whose duty it was to protect their towns, colonies and, eventually, states. Those bodies entrusted the militias with protecting the people against threats both internal and external. With only a small United States military at the time, militias were a vital part of America’s national defense. The most famous group of them at the time were the Minutemen. Highly trained colonists who fought against and ultimately defeated the British in the American Revolution.
In modern times, militias exist independently of the government. They tend to be comprised of disaffected citizens who view the government as having failed in its duties to protect, serve, and defend its people. As such, they often step in to discharge the rules and laws that they see the government as having failed to enforce. The majority of the groups have far-right ideologies and are paramilitary in nature. They often ascribe to conspiracy theories, many involving government plots to take away citizens guns and other basic rights. For an excellent overview of these groups, check out the article, The Militia Movement, on the Anti-Defamation League website.