What is the meaning of "off limits"?

I take a judgmental nibble. They’re slightly dry, which tells me she skimped on the butter. There’s also a severe lack of sugar, which suppresses the fruit. Rather than either lemon or blueberry, the muffin is the flavor of paste. I take a sip of coffee. It’s too strong. The bitter taste on my tongue bleeds into my words.
“We need to talk about last night—”
“I was a bitch,” Sam says. “You’re being all nice and I—”
“I don’t talk about Pine Cottage, Sam. It’s off limits, okay? I’m focused on the future. You should be too.”
“Got it,” Sam says. “And I’d like to make it up to you somehow