>>>super secret coded message sent by al qaeda? Nope. Just TV Techie-speak.
Transition is the way in which one scene changes to another, such as a dissolve (Scene A fades into scene B), a wipe (Scene A is replaced by scene B with a moving edge), or a key (Scene A has Scene B imposed on top of it, with certain areas cut out or "keyed" as determined by color or luminence so that parts of scene A and scene B are visible together)
Keying is used to place text on top of an image. There's no such thing as "clear" in video. Instead, certain areas are defined to be canceled out so that they disappear. Everything that's left is what can be seen, such as a big X.
A frame is a single image used to construct video. There are 30 frames in one second of video, so 5 frames is 1/6 of a second.
Transition begins after 5 frames of black, simply means that the keying effect (or dissolve or wipe) will commence after five frames of black video have passed.