If a rating changes significantly, then the share is said to have been rerated by the market.
A high rating may reflect growth prospects (or perceived growth prospects), earnings quality, market sentiment, or anything else that justifies a premium price.
An apparently high rating on one metric (e.g. a PE) may not be reflected by other metrics (e.g. price/book). In this case it is the rating on the metrics appropriate to the business and sector that should be used to judge whether a rating is high or low.