I don't understand the special ability a player gains after building the second stage of Alexandria side A. Can someone explain it? Thanks!
I assume you inquire about stage #2 of side A. According to the Rules, p. 8:
the player gains a resource of their choice from the 4 Raw material types [Stone, Clay, Wood, Ore] every turn. Clarification : this resource cannot be sold through commerce.
In other words, each turn you have a wild, exclusive raw material at your disposal, for free, just like if you had bought the Caravansary Age II yellow card. Other players may not buy this resource.