ns=sorted(sorted(s),key=lambda x:s.count(x),reverse=True)[0]

This line of code sorts the characters in a string 's' in ascending order, and then sorts them again in descending order based on their frequency of occurrence in the string. Finally, it selects the character with the highest frequency of occurrence and assigns it to the variable 'ns'.

Here's a step-by-step breakdown of how this line of code works:

1. `sorted(s)` sorts the characters in the string 's' in ascending order.

2. `sorted( ... , key=lambda x: s.count(x), reverse=True)` sorts the characters again, but this time based on their frequency of occurrence in the string. The `key` parameter specifies that the lambda function `lambda x: s.count(x)` should be used to determine the sorting order. This lambda function takes a character 'x' and returns the number of times it appears in the string 's'. The `reverse` parameter is set to `True` to sort the characters in descending order.

3. `[0]` selects the first character in the sorted list, which has the highest frequency of occurrence.

4. Finally, this character is assigned to the variable 'ns'.