"Safe" may mean one of the following:

• "Safe for work" posts that feature no content that would be inappropriate in a public, formal or controlled environment. These posts are covered by the rating:safe metatag, which isn't necessary on e621's SFW domain, e926.net.
• safe_(container), an object used to store and protect things such as money, valuables and sensitive documents. Also called a strongbox or a coffer.
• safeword, a code word or signal in BDSM used by participants to convey their physical or emotional state, usually when they are approaching a boundary that they don't feel safe or comfortable with, most often to stop the BDSM scene outright.
• safeword_(character), an anthro raccoon character with a BDSM-based theme.