WHAT DOES “CONNECTED WLAN CLIENTS” MEAN when stated in my broadband account with www.my3.ie?

What does the term “Connected WLAN Clients” mean when stated in my broadband  account with www.my3.ie ?  “Whenever I check it there are one or two connected WLAN clients using it?   Who are they and why are they non-consensually connected to my personal broadband account.    The MAC ADDRESS that is given for them is 4E:15:23:5B:02:1F.   Where are they located  and what does it all mean for me?