The meaning of "143" in online vernacular is "I love you." It is derived from the number of letters in each word of the phrase: the word "I" has one letter, "love" has four letters and "you" contains three letters.
The "143" slang term is typically used in SMS text messaging, but also in emails and on social media sites. The singer Musiq Soulchild brought attention to the use of "143" in his song by the same name. The lyrics explain that love is complicated enough and that simplifying it with a numerical phrase is an appropriate way to convey pure emotions.