Did you know that if you're talking about a work-related issue at a rally, like these held by OUR Walmart, you're protected from being fired?
Talking to someone else, or a crowd, about how lousy your work conditions are, or how little you get paid, is called a “protected concerted activity,” and it's protected by the National Labor Relations Act (NLRA).
Concerted activity also includes social media, so talking about it online is also protected. The important thing to do is talk about work conditions or wages with someone else, rather than talk about office gossip or personal stuff. Jut remember to put one of those two things in there: work conditions, or wages.
Not only that, but if you specifically “talk union” or discuss forming a union, or discuss improving your workplace, your speech is protected even more.
Did you know that, sometimes, the best places to learn about labor law are on websites for employers?
Who'da thunk it? While lawyers and unions will tell you some things, they don't necessarily want to give you advice that could cause you problems, so they want to talk. That's great, but what if you're curious about what really is protected concerted activity?
Searches will turn up sites geared toward employers. Sometimes, these articles go into great detail about what is, and what is not concerted activity. They will, for example, describe in some detail, situations where employees messed up. Study these, and you won't make the same mistakes.
You should also consult the NLRB's web page about concerted activity.