ESS is effective sample size – it is used analogously to the sample size you might use in a regular Monte Carlo estimator. Because in MCMC sequential draws are correlated, 1000 draws from MCMC isn’t the same as 1000 draws from a Monte Carlo estimator.
ESS is computed so that if you have an ESS of 570 from your MCMC sampler, that should be about the same as 570 draws from the distribution you want. This post links to the latest doc on our ESS: New R-hat and ESS