A thumbs-up gesture indicating approval.
It is the well-known symbol of approval and liking something — and the emoji, based on this gesture, is used online exactly in the same meaning.
For example, it may be used instead of "yes" as an answer to an offering the user likes very much; it may also serve as a sign that the user likes some ? Movie or photo or anything else, when it appears in a comment or review.
Thumbs Up was approved as part of Unicode 6.0 in 2010 under the name “Thumbs Up Sign” and added to Emoji 1.0 in 2015.
U+1F44D
? Thumbs up sign