fresh start with MDBT42Q , some specs before start toe code

I guess I need to convert the 0x1D to 29 using some java hex to DEC function ?

No... it's just the way that nRF connect is showing you the number I think. In reality in whatever variable you're using it's just a number like any other

description: "My Characteristic", // optional, default is null
I thought maybe I can write something there - but if it's no used , so never mind.

Yes, the description has no effect on the functioning of your code

what is 'heartrate' mean in the value section ?

That would be a variable that would contain the heart rate as a number. Like maybe 85 (eg 85 bpm).

where can I find a list of what I can put \ which values?

You just have to look it up online - the Bluetooth SIG used to have a webpage showing them but they've made them a bit tricky to find now.

You can just make your own UUIDs though for whatever you want to send, then it's up to you to interpret the result. Maybe check out https://www.espruino.com/About+Bluetooth+LE