What does Santiago realize about the wind, and how does this help him on his journey?

The wind is an inspiration to Santiago. If I read you correctly, your question is a reference to the text below:

That wind had brought the Moors, yes, but it had also brought the smell of the desert and of veiled women. It had brought with it the sweat and the dreams of men who had once left to search for the unknown, and for gold and adventure—and for the Pyramids. The boy felt jealous of the freedom of the wind, and saw that he could have the same freedom. There was nothing to hold him back except himself.

Santiago realizes that the wind carries with it the history of the past. The text tells us that the wind is also called the "levanter" and that it brought the Moors over. Although the author doesn't explain what this means, we can at least conclude that the wind brings inspiration to Santiago. The Moors invaded Spain in the 8th century. During this time, much of the Iberian Peninsula (where Spain is located) came under the rule of the Umayyad Caliphate. If you're interested, please refer to the link below for more information about this. The Moors essentially traveled from Africa to Spain to fulfill their ambitions.
So, it follows that, if the Moors can "search for the unknown...for gold and adventure" in a faraway land, Santiago can also. Initially, Santiago feels "jealous of the freedom of the wind." However, he soon realizes that he can have the same freedom. The wind (with its rich history) inspires Santiago to transcend his fears in order to realize his dreams.
 
https://www.newworldencyclopedia.org/entry/Umayyad_conquest_of_Hispania