Love is an emotion of strong affection and personal attachment. In philosophical context, love is a virtue representing all of human kindness, compassion, and affection.
As a tag on e621, it's kind of vague where to put "love"; but try to focus on images and animations where characters are overtly displaying their love for each other, i.e. kissing while cartoon hearts dance above their heads, saying "I love you" to each other, etc. Often but not always synonymous with romantic. Related tags
See also