There are 26 of these and they will translate the Al Bhed language one letter at a time for each one you pick up. I will not list all of them here at the moment as I'm by the start of the game, just the earliest couple you can pick up so you can get this quickly.
On the Al Bhed ship there is a door and to the right of it you will see a blue disc on the floor. Walk up to it to pick it up.
If you miss that then when you get to Besaid Village walk into the top left hut that belongs to the Crusaders. Keep pressing
as you walk in as this is slightly off screen and you will pick it up. Do this before you leave the village.
*** Spoiler - click to reveal *** If you don't pick this up and leave the entire Besaid area on the boat you will not be able to re enter the village for a long time after and are severely levelled up as Dark Valefor awaits
In the Bar in Kilika Port after you view the cutscene. Go back to the save point where you enter Kilika go into the first building you come to. You will see a man sat on the floor and the Primer is directly above his head