"To overestimate" is more broad and usually comes before a noun. It is most often used for quantities. It means you expect more than there actually is. Examples: I overestimated his appetite. He couldn't eat very much. He overestimated the quality of the movie. It was actually poorly made. She overestimated how many apples the bag could hold. The bag broke after 10 were put in. Don't overestimate the number of people who will come to your party. You are not that popular. Meanwhile, "to overrate" means to think something is better than it is. Usually in English we say something "is overrated", rather than say "he/she/it overrates". Examples: That song is so overrated, everyone loves it but it's not actually a good song. This movie is overrated, I saw it and I thought it was boring. I hope this was helpful and easy to understand :)

"I overestimated the amount of food I had to cook for the family dinner. We were eating leftovers for several days afterwards." "His cooking is OK but not special. I don't know why so many people go to his restaurant - he's overrated as a chef"