The dataset version is a number which definitively identifies the data currently published in a particular dataset.
The first time you publish data for a dataset, the dataset version is set to 1. If you later import new data (as a draft dataset), and go on to publish it, then the new data replaces the existing data and the dataset version increases.
If you own a dataset or have permission to reference it in a query, then you can check its dataset version using the Platform API. In particular, this lets you check whether the dataset you are referencing is the same as the one you referenced last time.