I need help understanding the code in the turn.py file, which is a part of the this project, please look

#!/usr/bin/python3
# File name   : car_dir.py
# Description : By controlling Servo,the camera can move Up and down,left and right and the Ultrasonic wave can move to left and right.
# Website     : www.adeept.com
# E-mail      : support@adeept.com
# Author      : William
# Date        : 2018/10/12
from __future__ import division
import time

import Adafruit_PCA9685

def replace_num(initial,new_num):   #Call this function to replace data in '.txt' file
    newline=""
    str_num=str(new_num)
    with open("set.txt","r") as f:
        for line in f.readlines():
            if(line.find(initial) == 0):
                line = initial+"%s" %(str_num+"\n")
            newline += line
    with open("set.txt","w") as f:
        f.writelines(newline)

def num_import_int(initial):        #Call this function to import data from '.txt' file
    with open("set.txt") as f:
        for line in f.readlines():
            if(line.find(initial) == 0):
                r=line
    begin=len(list(initial))
    snum=r[begin:]
    n=int(snum)
    return n

#import the settings for servos
vtr_mid_orig    = num_import_int('E_C1:')
hoz_mid_orig    = num_import_int('E_C2:')

turn_right_max  = num_import_int('turn_right_max:')
turn_left_max   = num_import_int('turn_left_max:')
turn_middle     = num_import_int('turn_middle:')

pwm = Adafruit_PCA9685.PCA9685()
pwm.set_pwm_freq(60)

def turn_ang(ang):
    if ang < turn_right_max:
        ang = turn_right_max
    elif ang > turn_left_max:
        ang = turn_left_max
    else:
        pass
    pwm.set_pwm(2,0,ang)

def right():
    pwm.set_pwm(2, 0, turn_right_max)

def left():
    pwm.set_pwm(2, 0, turn_left_max)

def middle():
    pwm.set_pwm(2, 0, turn_middle)

def ultra_turn(hoz_mid):
    pwm.set_pwm(1, 0, hoz_mid)

def camera_turn(vtr_mid):
    pwm.set_pwm(0, 0, vtr_mid)

def ahead():
    pwm.set_pwm(1, 0, hoz_mid_orig)
    pwm.set_pwm(0, 0, vtr_mid_orig)

What is pwm.set_pwm() doing in the methods:

• right()
• left()
• middle()
• ultra_turn()
• camera_turn()
• ahead()

Also, what does pwm.set_freq(60) mean? Please refer this link