So-called "love bruises" are typically the result of physical abuse. The victim is often shamed into thinking that the bruises are their fault. The victim make try to explain away the bruises by making them out to be an act of love. This is not healthy.
Often the physical abuse is preceded by emotional abuse and attempts to control or manipulate the victim, e.g. name-calling, telling him or her who they can be friends with, disabling the victim's vehicle to prevent them from leaving, etc.
Not the same as a love bite
After loosing her virginity to Edward Cullen in the "Breaking Dawn" book, Bella woke up with so many bruises that she started trying to figure out which ones would be the hardest to hide. This is not a Good Thing.
Twitard: "omg sparkle vampires iz so hawt! Edward can bite my pillows, break my headboard, and bruise my body any day!! They r just love bruises and it proves how much he loves her!!11!!1"
Twitard T-Shirt: "Bruises are love.... share the love!"
Me: *punches the Twitard in the face* "Just sharing the love"
Smart Person: "Bruises are not love. Love is respect."