What do people mean when they start an explanation with "Well, to be honest...."?

Usually, it’s one of two things. Either the speaker is about to tell you the truth at the risk of it getting out, or the person disagrees with your opinion and it’s a way to soften the blow.

When people say that to me, I’ll often interupt and say, “Whoa, waitaminute…are you usually not honest with me?”

I hear it on Judge Judy a lot. JJ is always, like, “Of course I want you to be honest! This is a court room! I’m a judge!”

They are about to tell you the whole truth, with all the ugly details, not just the palatable part they already told you. Some of this new information might be factual, but some of it might just be their opinion.

Besides what @Pied_Pfeffer said, I think the phrase also has a “vamping” quality to it, like “Well,” “So,” and the currently wildly popular, wildly annoying “Look” and “Listen.”

It means that usually they are lying to you, but in this case they will tell you the truth.

I find the phrase passive-aggressive. I hear it often, particularly from one individual, and it triggers my anxious anticipation of what is about to be said.

For me, it signals “I’m going to tell you something that you don’t want to hear, that is not based in fact, but that I’m framing as ‘the truth’ so you have to accept it.” It seems to be part of the endless rhetoric that slaughters the distinction of fact from opinion.

I guess I’m guilty of saying in response to someone pressuring me to tell them what I honestly think, when I really don’t want to because it’s going to piss them off.

For example, my daughter has a pit bull puppy. She knows I’m not thrilled with pits, but I’ve kept my mouth shut. Then she started pushing me to ask what I really thought of her little dog.
“Well, I like the dog. He sweet, really.” And he is.
“Yeah, but you don’t like pits, admit it!”
“Well, to be honest, no, I don’t. And you know that.”
And the lecture is on.

It is usually a phrase that people use when they try to soften to blow of being blunt.

It doesn’t mean anything.

We use words way too casually. We use words and phrases without considering what we may be communicating to the listener. Those words and phrases become crutches that we habitually turn to because we’ve become too lazy to articulate what we mean. We think that adding certain words or phrases make what we say more ________ , when it actually shows how inauthentic we are.

Here in the area of NY that I live in, it’s “I gotta be honest with ya.”

It doesn’t mean a conversation partner always lies to you and now s/he suddenly decides to tell the truth. It’s just a figure of speech dedicated to warning you that something you are going to be told can be unpleasant to hear. So you have time to get ready.