I am trying to connect the WiPy via BLE to an Android application, and I am at the stage where I am trying to understand how BLE works currently.

I was looking at the example advertising and connection code that is here: https://docs.pycom.io/chapter/firmwareapi/pycom/network/bluetooth/gattscharacteristic.html

Can someone explain to me how the code here works. My comments are how I interpret the code, based on what I've read about BLE, and basically my questions are as the comments state.

 from network import Bluetooth

 bluetooth = Bluetooth()
 bluetooth.set_advertisement(name='LoPy', service_uuid=b'1234567890123456')

 def conn_cb (bt_o):
     events = bt_o.events()     # Here we are checking if a device is connected.
     if  events & Bluetooth.CLIENT_CONNECTED:
         print("Client connected")
     elif events & Bluetooth.CLIENT_DISCONNECTED:
         print("Client disconnected")

 bluetooth.callback(trigger=Bluetooth.CLIENT_CONNECTED |  # Can someone explain this?
 Bluetooth.CLIENT_DISCONNECTED, handler=conn_cb)

 bluetooth.advertise(True)   #Enable BLE advertising

 srv1 = bluetooth.service(uuid=b'1234567890123456', isprimary=True) # what is "isPrimary"?

 chr1 = srv1.characteristic(uuid=b'ab34567890123456', value=5)

 char1_read_counter = 0
 def char1_cb_handler(chr):
     global char1_read_counter
     char1_read_counter += 1  # Everytime we want to read the first characteristic, increment.

     events = chr.events()
     if  events & Bluetooth.CHAR_WRITE_EVENT: # When client tries to overwrite characteristic?
         print("Write request with value = {}".format(chr.value()))
     else:
         if char1_read_counter < 3:
             print('Read request on char 1')
         else:
             return 'ABC DEF'  # If we try to read more than three times, we return this? Why?

 # What does the line below do?
 char1_cb = chr1.callback(trigger=Bluetooth.CHAR_WRITE_EVENT | 
                          Bluetooth.CHAR_READ_EVENT, handler=char1_cb_handler)

 srv2 = bluetooth.service(uuid=1234, isprimary=True)

 chr2 = srv2.characteristic(uuid=4567, value=0x1234)
 char2_read_counter = 0xF0
 def char2_cb_handler(chr):
     global char2_read_counter
     char2_read_counter += 1
     if char2_read_counter > 0xF1:
         return char2_read_counter

 char2_cb = chr2.callback(trigger=Bluetooth.CHAR_READ_EVENT, handler=char2_cb_handler)             
 #What does the line above do?

Thank you all for your help and time.