It is Slicing. s[a : b] here a is index and b is negative index. After slicing it will return value from index a(inclusive) to b(exclusive).
It is not necessary for b to be negative everytime. You can check out docs: https://python-reference.readthedocs.io/en/latest/docs/brackets/slicing.html#slicing

thx i did it

Please see if you can find anything helpful in this paragraph: https://docs.codewars.com/training/troubleshooting#works-but-no

True

My solution is correct in 98 of 105 cases and i cant find the issue :C.
Any tip?

ok

I get it Thank you