ES stands for Engineering Special. Generally speaking an ES is a rollup of fixes that are not yet available in any public release. Additionally, the fixes have not gone through the more complete testing (e.g. regression testing) that a formal release has.
Most products have an ongoing changelog document that explain what defects are covered by each ES. You can use that to look up the details of each defect in the bug toolkit if the bug has been marked public. You typically have to obtain the changelog/release notes from the Cisco support resource you are working with. None of the ES information is public typically.
PS- Moving this thread to the Contact Center area might provide you more on-topic responses.