The full form of 143 is “I Love You (letters in each word)”.

143 means “I (1) love (4) you (3).”. The numbers represent the number of letters in each word. Other versions of this include:

381 = I love you (3 words, 8 letters, 1 meaning)
721 = Love you (7 letters, 2 words, 1 meaning)
831 = I love you (8 letters, 3 words, 1 meaning)

143 is a form of shorthand typically used in texts or instant messaging. It also allows the speaker to say “I love you” without suffering the embarrassment felt by some (particularly men) when saying those words.

See sometimes words may get in the way Of things that you really really mean to say So I wanna out this time to make sure that how I really feel Gets through to you Cause listen
There aren’t many more ways and words that I can say,
So I choose to tell you how I feel in a numerical way,
Got so tired of trying to find the most complicated way,  To say it to you so I simplified and broke it down
To 143 – and that means I love you