What is the meaning of "it is safe to say"?
It just means that you're pretty sure it's correct, in the circumstance where something related could have been wrong.
People generally say this phrase when there is a chance they are wrong, and they may compromise by (implicitly) confining themselves to certain conditions and then find that it "is safe to say . . .".
For example, you might say something like,
"Well, it is safe to say that no sane person would willingly go out and kill someone else."
Had you said,
"No one [ever] would willingly kill someone else",
you would have been wrong, because that kind of event has definitely happened before.
So, in your "it is safe to say that . . . " phrase, you've implicitly confined yourself to the conditions that the person is sane.