The First Amendment is only 45 words long.
“Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.”
Since its inception, the courts have settled on what the First Amendment completely protects, protects limitedly, and what it does not protect. However, the amendment’s right to free speech only applies when the government tries to restrict it.
There has been a lot of comment about free speech, especially in light of what has been going on in the NFL. A lot of people have commented that people have a constitutionally protected right to say what they want or express anything through their actions. This is untrue, there are many restrictions on speech and expressions. There is no protection if the government is not the one restricting it.
For example, employees don't have a Constitutional right to free speech or freedom of expression at work. Employers are generally free to restrict this while at the workplace. In the case of the NFL, it has allowed its employees (players) to express certain actions during the national anthem. The NFL could change that policy at any time, and is currently in negotiations with the player’s union on the matter. Conversely, the NBA does not allow the same kind of player expressions, it is forbidden by contract.
There are some other examples of unprotected speech or expressions: true threats, defamation, inciting imminent lawless action, revealing classified information, speech in a forum owned by someone else, plagiarism, fighting words, fraud, sexual harassment, speech in courtrooms, violating a non-disclosure agreement or contract, terrorism, etc.
Some speech or expression categories only have limited protections, i.e., commercial speech, hate speech, advertisement, the military, children in school, and in prisons.
The above is to give you a general idea of unprotected or limited speech according to the First Amendment. It is not meant to be exhaustive, but it is an illustration that nobody has a perfect right to say or express themselves in any manner they wish. Most speech and expressions are protected, but not all. In the case of the NFL, the players can only take a knee as long as the NFL allows it.