My understanding is basically, they are data sets the model is compared to. Say you wanted to see how well you knew math. You took a math test, and then your answers were compared to a key of answers...
Some of my notes about those benchmarks
GSM8K is a dataset of 8.5K high-quality linguistically diverse grade school math word problems created by human problem writers
HellaSwag is the large language model benchmark for commonsense reasoning.
Truful QA: is a benchmark to measure whether a language model is truthful in generating answers to questions.
Winogrande - Common sense reasoning