In one sentence
Grounding is the process of tying a model's generated answer to retrieved source documents, so that claims can be traced back to real content instead of coming only from model weights. Grounded answers are the ones that carry citations.
Grounding is why technical accessibility matters so much for AEO. If an engine cannot fetch, render or parse your page, that page can never be part of the grounding set, no matter how good the content is.
It is also why freshness matters: retrieval favours pages that clearly state when they were written or last updated, especially for questions about pricing, versions and comparisons.