An audit is an educational term in the United States for the completion of a course of study for which no assessment is made or grade awarded. The grade of "audit" is awarded to those who have elected not to receive a letter grade for a course in which they are typically awarded.
In this case, 'audit' indicates that the individual has merely received teaching, rather than achieved a given standard. The term 'audit' is Latin, translating as 'he hears'; in other words, the student has experienced the course but not been assessed.
This technique is often employed by individuals who wish to take a specific course without the risk of under-performance resulting in a poor or failing grade. This can be helpful when reviewing a long-unstudied subject, when first beginning the study of a discipline wherein one has little experience or confidence, when taking a course merely for enjoyment with no need or desire of academic credit, et cetera.
Auditing is generally an option at institutions of higher learning (colleges, universities) rather than grammar school (K-12).