A rating is a valuation or rank on a scale. Hotels are often ranked from zero to five stars, so a fancy hotel with loads of amenities (like a pool and plush bathrobes) will usually have a five-star rating.
A movie gets a rating based on which audience it’s appropriate for – as in, rated G for kids or rated R for adults. When a TV show gets good ratings, then a lot of people watch it. Before you buy an expensive item, like a bike or an appliance, you can read its ratings online, and once you've tried it out, you can provide your own rating.