Could you explain what does strarr[index:index +k] do

I found something Helpful suggestion from early comments and just want to repost what he commented
It is pretty helpful to understand that's why..
Crd --- Tigerdevon
Example:

longest_consec(["it","wkppv","ixoyx", "3452", "zzz4zzzz9zzz"], 3)
The three options:

1) [0]"it"    [1]"wkppv" [2]"ixoyx"        ; "itwkppvixoyx"
2) [1]"wkppv" [2]"ixoyx" [3]"3452"         ; "wkppvixoyx3452"
3) [2]"ixoyx" [3]"3452"  [4]"zzz4zzzz9zzz" ; "ixoyx3452zzz4zzzz9zzz" 

k(int) is the reason why there are 3 strings per answer

The answer is:

3) "ixoyx3452zzz4zzzz9zzz"

No, because that's not what you have to do.

Your task is to return the first longest string consisting of k consecutive strings taken in the array.

You have to return the first one if there are more than one with the same length.