What does Hrothgar caution Beowulf against?

I agree with the other person - pride is something that Hrothgar cautions Beowulf against. Excessive pride (also known as hubris in Anglo-Saxon poetry) is not a desirable characteristic that a leader/warrior should have because it will lead them to their demise in the end. This is what happened to Beowulf ultimately - he thought he could defeat the dragon in his old age because of his hubris, and because of it, he died.

Hrothgar cautioned Beowulf against one of the most important things ... Pride