Simply, you can think of this "Episode 000" like a trailer of "Full Metal Panic! The Second Raid" It doesn't say too much, it's something to get fans exited before "The Second Raid" (TSR). Also introduces you to the new characters. It's something worth to see. It's as worth as seeing a trailer. Keep in mind that only have a duration of around 5 minutes, so you don't lose anything.