The exact nature of serfdom in the Aztec world is not entirely understood, but many anthropologists and historians have offered theories and explanations for it.
Serfs (Mayeque) were commoners who worked on state or privately owned plantations and functioned primarily as agricultural laborers. They did not own property. Rather, they were permitted to live on a plantation or estate if they worked the fields and contributed a (probably significant) percentage of their crop to the owner. Serfs can be differentiated from commoners (Macehualtin) because commoners owned property even if they were permanent agriculturalists.
Some sources that I have read suggest that serfdom was hereditary, meaning that a serf’s children were invariably the same. This is questionable, especially considering how meritocratic Aztec society was when compared to the rest of Central Mexican history. The highly inclusive, militaristic nature of Aztec society necessitated adequate rewards for commendable service in the military. Commoners could become nobles, and nobles could become wealthy and respected through warfare. Excluding a significant number of the population from these rewards would have demoralized much of the army and reduced its effectiveness.
Aguilar-Moreno suggests that serfs were technically free, but worked on plantations out of choice. If this is true, its probable that they remained on the plantations because they were unable to afford their own property.
Anthropologist Ross Hassig argues that Aztec serfs were likely an example of an internally displaced people. Oftentimes, land and estate acquired by the Aztec empire through conquest was given to important nobles as a reward for military service, civil service, etc. Many of the individuals who used to own that land likely stayed there for economic reasons, despite the fact that ownership had been taken from them and granted to another.
It is known that, by Aztec times, commoners were given access to the nobility based on their achievement. I would argue that, perhaps, serfs were given the same opportunities. Serfdom would therefore not be hereditary and a serf could theoretically rise out of the peasantry and elevate their social status. There were, however, significant factors which stood in their way. They lacked the money to pay for their own property, lacked the training necessary to excel in combat, and were unable to participate in the more lucrative economic positions that urban centers could offer.
I will attempt to hypothesize a chronology about how Aztec period serfdom developed: In periods with relatively low conflict, serfs operated as a completely separate class from urban commoners or the nobility. Gradually, as conflict escalated and more soldiers were required to participate in campaigns, the ruling class tapped into the peasantry to swell their armies. Because repression of a class became more difficult as it was given an increased role in the military, the nobility was forced to begin blurring the lines between serfs, commoners, and nobles. By allowing some social escalation and supplying adequate rewards, the nobility ensured that the mayeque remained content.