7 wonders Alexandria side A special ability
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.