Huevos de tortuga are what their name says: turtle eggs. They're taken from the nests at Ostional during the first part of turtle season, and sold in seafood stores or restaurants.
A turtle egg is eaten raw. At the most, you add a bit of tabasco and a shot of Guaro Cacique. People who eat them remark on the texture and the sensation as it's going down.
Turtle eggs are not illegal, especially if sold in supermarkets or seafood stores. During turtle season a part of the eggs are collected to avoid overpopulation in the nests and make it easier for the hatched turtles to move about. Turtles arriving after the first day will usually damage existing nests if they can't find space for their eggs, so some nests are cleaned out to avoid this.
There's an allowed collection weight per harvester, which is calculated so that it doesn't harm the turtle population.