If the characters info-dump, through dialog or thoughts or what-have-you, exactly what the reader needs to know, it's a bit jolting. Perhaps it's just that it's impossible to make the motivations for that so neatly. Some things ought to be implied between the lines, and some things needs to be explain for the characters' own reasons, even though the audience would reasonably be expected to know them. Unless you go and get meta, which I will not delve into.
This gets ticklish in first-person. One expects a narrator to explain what is needed, but then, very few first-person narrators intend the audience to be the actual folks who buy the book. An account written to the narrator's grandfather should not have a family tree -- unless the narrator says something like "this is the family tree Father gave me, and if he got anything wrong, this is why the relations are not treated properly in my account or actions." Even if it's written for far-off foreign lands. Especially so, perhaps. The narrator will explain what he thinks such exotic people will need to know. Which will reveal much of what he thinks of far-off foreign lands.
There's a lovely bit in Operation Chaos. The narrator is attempting to broadcast a warning from his dimension, and he notes that he will probably get someone in an alternate world much like his -- someone who will remember Newton, and probably Einstein. He urges the person to listen even if he already knows who won World War II, because better too much than too little.
Then he starts with an incident in World War II, and lo and behold, we don't know who fought World War II.