/j is part of a tone/emotion tagging
system over text. It means joking, hence what the ‘j’ is standing for. So this tone
tag is placed after a joke that was sent over text.
It is one of many ways to convey what the tone, lightness, seriousness, and humor is used in a text. You place it at the end a message.
This
system was created for and by neurodivergent people who struggle to read the intended tone of something said over text, but everyone can and should use them. There are many others such as /srs (serious), /s (sarcastic), /lh (
light-hearted), /hj (half-joking), etc.
/srs