The illusion of truth effect is where something which is familiar is more likely to be believed. If you hear somebody make a statement you may have no idea if it is true or not. However, if you hear that statement again a week later it will sound familiar to you and you will be more likely to believe it.
Hasher, Goldstein and Toppino, in 1977, asked students to read 60 obscure, but plausible statements of fact. They had to rate how certain they were that each fact was true or false. They went through the same exercise three more times, at two week intervals. Some statements were repeated and some were not. By the end the students were far more certain of the truth of statements that had been repeated than of statements they had only read the once.
The participants did not necessarily remember reading any given statement before, but there was clearly an effect of memory going on. Where a past experience has an effect even though the person does not remember that experience, it is called implicit memory. The memory is implied by the effect it has (such as causing the student to believe the statement).
Find out more about how the mind plays tricks on you and how your memory works by reading my books, Bias Beware and Memory Matters.
Hasher, L., Goldstein, D., & Toppino, T. (1977). Frequency and the conference of referential validity. Journal of Verbal Learning & Verbal Behavior, 16(1), 107-112.