Is this grammatically correct? And what’s the meaning? “This is the sweetest song you would hear today, and I hope it makes your evening”

Yes, the sentence is grammatically correct. The meaning of the sentence is that the speaker is describing a particular song as very pleasing or heartwarming ("the sweetest song") that the listener will hear on that day. The speaker expresses a hope that listening to this song will positively impact the listener's evening, perhaps by making it more enjoyable or uplifting.