@zeccano Hello and welcome to Qoto.
Were you readign that post you wanted to reply at another instance, another browser tab?
If so, it can be responded to there -- but that server has to confirm that you are a legit, registered user somewhere. To avoid spam.
So it pops up a window to ask for your home instance, and once it's confirmed, you can reply there.
You can also do it from here -- use the search box and input a user name or some text that is unique to that post. Once found, you can reply immediately.
@zeccano Hoem instance means the server where we have our own account.
For both of us, it's qoto.org
You can have more than one, if there's a good reason. Although it can get more confusing, at least in the beginning.
Some instances have different themes, and you might be able to talk about things with people with similar interests, get better insights.
some are General, all topics go, as long as the server rules are respected.