Outsiders are beings that, as the name implies, exist outside of our reality and understanding. From investigations we have found they exist outside of all realities and are known across them. Any existence formed of matter runs alongside the reality of these beings, though they don't all interact with each other.
These beings have often been described in various ways - one of the most common being 'eldritch', or commonly 'Lovecraftian' named after a man who wrote much of his works about these very subjects or ones like them. They have the ability to communicate and influence creatures in any (that is all) parallel realities to lead them towards their own unknown goals. These usually involve bringing the outsider into our reality.
The entry of an Outsider into our reality has been labelled Event Envy, the end of our reality as we know it. Ours and theirs cannot coexist and should not cross.