This is the best way I can think to explain the "s" in "at the undertaker's" right now. Your name is Yasin. I could say that I am "at Yasin's house" or "I'm at Yasin's". It means the same thing. So, "I'm at the undertaker's" means that they are at the undertaker's place of business or residence. Hope that helps.

The 's means possession, in this case it means at the undertakers place... a place the undertaker owns.. For example, I'm at John's , means I'm at John's house or a place John owns. Another example is... She is Matt's friend ... the apostrophe gives possession. The friend is Matt's. It is the dog's food. It is my mom's birthday. They are all meaning ownership.

it means possesion