In the absence of language, it might not be able to tell what the cat was thinking, but it might be able to tell that there was a cat nearby. Think of it as being analogous to sound. We might be confused by a lot of people talking at once, as far as the meaning of what they were saying was concerned, but we would be able to detect a crowd of people all talking at once. Creatures without language, or unable to understand our language, would not be able to get information about what we were saying in any case. But if they heard a lot of human voices, they would at least know that there were a group of humans around, and be able to tell which direction it was in.
Going back to the rat, merely being able to detect the cat's brain processes might be enough to enable the rat to avoid an area where a cat was waiting in ambush. It wouldn't need to know that the cat was planning an ambush; just that the cat was there.Again, think of it as being analogous to sound. Beings without hearing might think it would be overwhelming to be constantly hearing different sounds, but it certainly has survival advantages.