By default, worlds are run in -ultrasafe mode. However, a .dmb file in a directory by the same name (eg world/world.dmb) will be run instead in -safe mode. When an unauthorized file or shell command is executed from Dream Seeker, the user will be prompted for authorization. Dream Daemon allows manual setting of the security before the world is executed. Note that the dantom hosting service always runs worlds as -safe.
How will all of this affect you? It probably won't even be noticed. Most worlds will be run in -safe mode, which means that they can have full file access within the current directory or below. Only worlds which are run in an incongruent directory (such as the desktop) or which access other files in the filesystem will result in security prompts, and of course those may be bypassed manually.