X is how you sign your name when you don't know how to read or write.
When you come from a first world country it's easy to think everywhere has the same education and opportunities as you did. But here especially I've learned that's not always true. According to United Nations numbers for 2012, literacy rate for 15-24 year olds is 88%, so it's getting better. But unfortunately, when we're talking about the older generation, ESPECIALLY the older generation in rural areas, it's more common that not that they don't know how to read.
I work a lot in a rural area right outside Granada called Los Cocos and El Guayabo and my generation and older have been very open with me explaining they're not literate. They explained that for the older ones, it was more needed for them to work in the fields and help their family than go to school. And for my generation, their schooling was interrupted by the Revolution and just never really got back on track. But this simple, humble people are so willing to learn and teachable; it's been a pleasure to work with them. Illiterate certainly is NOT synonymous with stupid.