Online etiquette is about applying basic respect and consideration to digital communication.
Written messages can easily be misunderstood because tone, facial expressions and body language are missing. Before sending something, consider whether the wording could reasonably be read differently from how you intended it.
Disagreement is part of online communication. You can challenge an idea without attacking the person behind it. Avoid unnecessary insults, threats, harassment and repeated unwanted contact.
Remember that forwarding, screenshotting or reposting another person's message can change the context and audience. Consider whether you have permission to share it and whether sharing it could cause unnecessary harm.