This is a C function named "_strlen" that takes a pointer to a string as an argument and returns the length of that string. The function uses a loop to iterate through the string and increment a variable "len" for each character in the string until the null terminator is reached. The null terminator is a special character represented by '\0' that marks the end of a string in C.

The function has a return type of "int", which means it returns an integer value. The function also has a parameter "str", which is a pointer to a character array representing the string.

The function first initializes the variable "len" to 0. Then, it uses a for loop to iterate over each character in the string pointed to by "str". The loop continues until it reaches the null terminator character, at which point it terminates. Inside the loop, the variable "len" is incremented by 1 for each character in the string. Finally, the function returns the value of "len", which is the length of the string.