I worked out early today, I did my water aerobics and my treadmill walk for 30 minutes.

I've noticed when I first started walking on the treadmill earlier this month my heart rate ( at my fastest speed)reached the 150's then the next week it went down to the 140's, now it's down in the 130's
does that mean my heart is getting stronger?

Does it mean I am getting in better shape?
I am guessing that is what it means, I don't know for sure but im pretty sure it's a good thing.