I read your topic and thought "I am the last person on earth that should give an explanation on what speaking the truth in love means".
But since I do know, or I think I know, I'll explain and we'll all just go along under the understanding that this is something GirlieGirl really needs to work on.
It comes from Eph 4:14-16
As a result, we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming; 15 but speaking the truth in love, we are to grow up in all aspects into Him who is the head, even Christ, 16 from whom the whole body, being fitted and held together by what every joint supplies, according to the proper working of each individual part, causes the growth of the body for the building up of itself in love.
More or less, speak truthful words as Jesus would speak them. When he convicted those of sin, he didn't condem them harshly - pharisees aside. When it came to sinner, he reproved and corrected with a gentle hand. His words revealed a sinly condition and guided the sinner lovingly towards the right path. If one were to speak the truth in love, they would be saying the truth gently and with the altruistic motivation to guide the sinner away from sin - instead of with the motivation to condemn, cast judgement, or outcast.