"What is the meaning of"
The initial course date would be the date you received the first vaccine.
The date of the booster would be the date you got the follow up vaccine.
You see some vaccines like small pox, measles and yellow fever you need to have more than one shot for you to be fully inoculated against these infections. When you get the first shot, the nurse or doctor tell you when to return for the booster shot.
Hope this helps.