Meaning
A closed fist with the thumb pointing up, usually in a default yellow that can be changed to other skin tones. It is the all-round sign of approval: yes, good, agreed, got it, nice work.
It is a quick, versatile reply: βMeeting moved to 3 πβ Its very brevity is the catch. Many younger users see a lone thumbs up as cold, dismissive or even passive-aggressive, especially as the reply to a long or emotional message. For a warmer tone, add a word or two, or pick a smiling face.