A lack of filter means saying whatever is on your mind without considering politeness, appropriateness, relevance, or respect for the situation. People who have no filter may use profanity when they should not or bring up sensitive topics such as religion, politics, sex, body parts, or body odors in ways that offend others. Human communication follows social norms, and these norms discourage creating unnecessary controversy, embarrassing people, or discussing highly divisive topics in casual conversation. When someone ignores these norms, they may be seen as awkward, weird, or socially unaware, and others may feel unsafe or uncomfortable interacting with them. Appropriateness is a skill that can be learned by thinking before speaking, considering the audience and situation, and asking whether something is respectful, relevant, and suitable to say.
Everyone has various filters they use depending on the audience. Some may use stronger filters, some may barely ever filter. But everyone does it.
It is smart to frame communication around the audience as without considering the listener, you may not be communicating at all and instead are just making noises that fall on deaf ears.