Stockholm syndrome is a psychological response to a stressful situation wherein the victim, despite the ordeal they are put through by the victimiser, shows signs of loyalty towards them. By being cooperative, the victim reduces their likelihood of being physically harmed, and in cases of isolation, with only the victimiser's point of view available, the victim may interpret the lack of abuse as kindness. The victim focuses on the victimiser's "good side" in order to mentally cope with the situation. The victim is dependent on the victimiser for everything, including their so-called kindness, and begins to see them almost like a parent figure, to whom they become emotionally attached. The victim does not do this deliberately--their feelings of attachment are quite real to them, and they may never lose them.
The syndrome is named after the Norrmalmstorg robbery of Kreditbanken at Norrmalmstorg, Stockholm, Sweden, in which the bank robbers held bank employees hostage from August 23 to August 28 in 1973. In this case, the victims became emotionally attached to the hostage-takers, and even defended their captors after they were freed from their six-day ordeal.
In fanfiction, particularly Hurt/Comfort fics, the syndrome often occurs but is rarely named as such. A typical case of unintentional Stockholm syndrome occurs when one character rapes another character, after which they fall in love. Real-life rape victims will usually attest that this does not happen (if they attest at all). If it does, it is not real love, but a defense mechanism enabling the victim to survive in a dangerous situation.