Okay! Imagine your heart is like a drummer in a band, keeping a steady beat. "Sinus rhythm" means the drummer is doing a good job, playing a normal and steady rhythm.
Now, a "1st degree AV block" is like when the drummer has to wait just a little bit longer before passing the rhythm to the other musicians. So, the beat still goes on, but it's a tiny bit slower than usual when it’s shared with the rest of the band.
In simple terms, your heart is still beating regularly, but there’s a tiny delay before the signal gets to the heart's pumping parts. It's not too serious, and many people with this can still play just fine!