Here's a breakdown:
* "al" means "please"
* "st" is a contraction of "als" which means "if"
* "ublieft" comes from the verb "bevelen" which means "to order" or "to command"
So, literally, "alstublieft" means "if you please" or "if you would be so kind".
Here are some other, less common ways to say "please" in Flemish:
* "gelieve" (more formal)
* "doe mij de moeite" (more informal, literally "do me the trouble")
* "mag ik" (for asking for something politely, literally "may I")
Remember that "alstublieft" is the most widely used and understood phrase.