food is actually a very complex subject and it varies greatly from region to region in the country
Traditional mexican food could be known as the food of the region BEFORE the spanish conquest.
A common element in traditional mexican is the basis of corn in the cuisine. Before the spanish came, corn was the equivalent to the wheat of our modern culture. After the spanish conquered, corn was put down as non-nutritious and downright garbage, so you can imagine how greatly this affected the cuisine after that. From masa used for anything from tortillas to fillings. Or the husks of the corn used to wrap various items in the central and northern regions.
Chilies are also a very important part of mexican cuisine, the most important being guajillo and ancho. Instead of relating them to spices like we do here in the US, they are actually a culinary vegetable in mexican. Other good chilies to know are: chipotle, habanero, chilies negros, and cascabels. Each of them has a unique taste and heat.
Chocolate is also a staple of traditional mexican. Beans were used as currency in many cases in ancient culture. The origin of the word as we know it today is aztec, and chocolate wasn't known in the modern world until it was brought back to Spain in the late 18th century.
The application of these basic staples in mexican cuisine vary from region to region as do culinary tools in mexico.