Ratator Pest is a mouse exterminating company. So far, all that's been seen of them is their online ad.
Jon had discovered their website on the his computer a few weeks ago. After discovering the house is infested with mice, he showed Garfield an ad for them. Despite their advertised effectiveness, they were also expensive. Jon didn't plan on calling them yet, expecting Garfield to get rid of all of the mice first. Otherwise, Jon will call the company and use Garfield's food budget to pay them.
Garfield became worried about Squeak and the other mice (as well as himself). He tried to convince Squeak to get all of them to leave, but to no avail. That is, until he showed Squeak the online ad. Squeak was, of course, horrified, and thus willing to do as Garfield says so Jon won't hire them. When Jon got back, the mice are still in the house. But before Jon can call Ratator Pest, Garfield managed to get all the mice into a bag, which he tossed outside. Jon then decided to reward Garfield with takeout from Luigi's Lasagna.