Can someone explain “mom brain” to me?

My friend and I were both diagnosed with infertility, she was able to get pregnant the very first month after this diagnosis by taking progesterone (yay!) and with a snap of a finger she stopped asking me about my infertility. Some have said “maybe she doesn’t want to rub it in your face” but all we talk about is her baby so I don’t think that’s it. Others have said “she has mom brain give her a break!”. I’ve obviously never been a mom yet, but curious what does “mom brain” do to you? As an outsider, it seems like an excuse to forget about everyone else and be flakey. Now, I get that hormones are nuts and you can become more forgetful and you have a lot going on... but did you really forget to think about your friends or do you just not have time or care anymore?