A content warning, or better known as a trigger warning, is defined as: "a statement at the start of a piece of writing, video, etc., alerting the reader or viewer to the fact that it contains potentially distressing material (often used to introduce a description of such content)". Using content warnings is a very well established practice in social justice, safe space communities.
A content warning's purpose serves to give people, especially trauma victims and survivors, the option of proceeding with caution or to not proceed at all. They are essential in a sense that they provide safety. These warnings can save a person from reliving their traumatic experiences or save them from an anxiety or panic attack. They are a means of considering others, while one speaks of a potentially triggering topic. Triggers are unique to everyone, as well as non-triggers. What you are desensitized to may not be the same for others, whether or not you've endured elements pertaining to that topic.
For instance, I'm a part of an online community, where I stay up to date on current events, as well as expelling frustrations with our power imbalanced society with other group members. It's a pretty safe area, where I feel secure in confiding with members with similar experiences. With the group being a social justice friendly atmosphere, trigger warnings are a vital rule. Sometimes in the group, there are members who post topics with severe nature without a content warning, which can cause a member, or members, to become very upset. Being given the option to proceed reading the post with caution, or to not proceed, is essential.
There are times when black members - myself included - cannot stand to see another article plastered on social media about another black person being unjustly murdered by law enforcers. It isn't that we do not care to discuss these unfortunate events or want to become ignorant to these happenings, it's just that we hear about it so much and just want a break from it all. It's the most relieving thing when someone provides a content warning of a post with: "police brutality, anti-blackness", to give us an option of proceeding carefully or to skip over it, for our own safety. It isn't the best thing to be slapped in the face with a video recording of a black person dying in a puddle of blood from a trigger happy officer without a content warning.
Content warnings, for these very reasons, are essential in providing safe environments for trauma victims, survivors, and marginalized communities.