To quote from the grammar notes for the section about da/de:
Da and De
Use da when you're talking about quantity. Use de when talking about a property or possession. The direct object -n ending (accusative) is not used after da or de. For example:
- glaso da vino -> glass of wine (quantity)
- glaso de vino -> wine glass (property)
- la porko de la viro -> the man's pig (possession)
- manko de mono -> a lack of money (property)
Therefore, the «da» indicates that you're speaking about some quantity of the noun that follows.