Simple question - messenger upgraded the security of this chat. Messages and calls are secured with end-to-end encryption - From the other side

Hello, everyone. I know what the function itself means, but practically I have a question. For example, I have a correspondence with person X. We have written something before days , and the other day I see that in the chat at the very bottom appears this caption "messenger upgraded the security of this chat. Messages and calls are secured with end-to-end encryption ? What does this mean , that in practice the other party/ the other person has made it so that only "me" and "he/she" can see our chat or what, I can't understand ? I haven't written messages after that , does that mean if I write now, they will be delivered or is it some form of "restriction" to not chat with the person?