Because /t/ to /m/ is difficult, the “t” of “it” is pronounced with your lips instead of your tongue. It’s important to keep your lips closed between the two sounds. We have the same problem with /d/ to /m/, so make the “d” with your lips also, remembering not to open your lips until you let the air out of your nose for the /m/ sound. This will take practice, and feel strange at first, so listen and copy as many times as you need to.
The phrase means, “It was the best thing that happened to me today”. Maybe it was that girl or boy who smiled at you, or that lottery win, or something kind somebody said.