Many people toss around the phrase "perfect love and perfect trust" in the Wiccan community. They sometimes use it as a defense when they are corrected by someone else, or bring it up as an arguement when they don't like what people are saying, as in, "I am TOO right, you're all being mean, what about perfect love and perfect trust?"
"Perfect love and perfect trust" appears in Lady Gwen Thomson's Rede of the Wiccae, which was first published in 1975, although the phrase itself is older than that. In Thomson's poem, she states, "Bide the Wiccan Laws ye must, in perfect love and perfect trust." The notion of perfect love and perfect trust is a simple one: that you are safe within the circle of your coven's practices. To stand in a circle with someone is to share an intimate -- and often vulnerable -- space with them, and it can only be done effectively with someone whom you trust implicitly. By that same token, if we are able to love our coven brothers or sisters, we are able to trust them with our safety and our lives.
Prior to appearing in Thomson's work, the phrase has been documented as showing up in Gerald Gardner's early BOS. Before that, the origins of "perfect love and perfect trust" are sketchy at best.
In some traditions, the phrase is used as an entry to the ritual space, as in, "How do you enter this circle?" with the reply being, "In perfect love and perfect trust."
Keep in mind that "Perfect love and perfect trust" is not a rule followed by all Pagans or Wiccans. Also remember that among those who do follow it, it is often applied only to coven members, and not to random strangers.