How do you differentiate between
He is playing in the field.
He is playing on, in the field.
Edit:I had seen most actually 'Virat playing on ,in the field' below an image.
I agree with the others, "He is playing on, in the field." means someone continued playing in the field, with an implication that there was a reason they might have stopped. The sentence is perfectly valid, although it sounds a little awkward and rare.
Example 1: The child was allergic to pollen, but he played on, in the field. This is valid, except "playing on" almost always refers to sports.
Example 2: The footballer was hurt, but he played on, in the field. This is valid, but it sounds very strange to specify "in the field". The only place a sportsman could continue to play is on the sports field. I think the unusual/awkward phrase is explained by the fact that this was an image caption. The author was probably consciously constructing a caption for people who couldn't see the image. The image is a sportsman who stayed in the game, the awkward and redundant "in the field" was consciously added to create the mental imagery for people who couldn't see the picture.
"In the field" is where it is.
To "play on" means to continue to play, especially despite circumstances in which it might be better for play to end.
A dense fog has rolled in, but Manchester City want to play on.
He has strained a muscle, but is playing on.
He has strained a muscle, but is playing on in the field, not as catcher. [baseball example]
The second one - "he is playing on, in the field" - doesn't read very well. Did you see this sentence somewhere?
The only way I can imagine that sentence being used is if on is being used in the sense of "continue". We can say things like
He played on
to mean "He continued playing", or
to mean "Please continue." So "He played on, in the field", could only mean "He continued playing, in the field." It does not scan very well, due to the two prepositions in a row, but it is grammatically correct.