Rencor means deep resentment or a grudge, especially after being hurt. In the song, the singer says "yo no le guardo rencor" meaning "I don't hold a grudge against her," a powerful expression of forgiveness despite all the suffering she caused.
In “Si la Ves,” the singer asks someone to pass a message to an ex: “Si la ves” [If you see her], “dile” [tell her] that he suffered when she left, but has finally forgotten her. He wants her to know that his dreams came true, his songs are playing on the radio, and he is doing better than ever. The line “ya puedo cantar” [I can sing now] works in two ways: he is succeeding as a singer, and he has recovered enough to feel happy again. He even says “la felicidad se me puede notar” [you can see my happiness], which is a wonderfully direct way to announce that the heartbreak is no longer in charge.
The message is not entirely bitter. He thanks her for what she did for him, admits that she caused him suffering, and says he holds no grudge. He has a new love who gives him courage and unconditional love, so “ya no me quema tanto el frío de su ausencia” [the cold of her absence no longer burns so much]. There is also a surprisingly generous blessing: he hopes his former partner finds a great love and is happy, even though she was not happy with him. Still, he tells the messenger not to mention that he once thought about looking for her and forgiving her. That little denial makes the song feel human: he says it no longer matters, while making very sure we know it once did.