If something happens subtextually, it means it happens in a hidden or not obvious way, like in a secret message.
When something happens subtextually, it can make people feel like they are part of a special secret or that they are very smart for understanding it.
Examples:
The characters in the movie communicate subtextually.
She subtextually hinted that she didn't like the idea.
The message was subtextually hidden in the painting.
The cat subtextually told me it wanted more food.
The treasure was subtextually hidden in the old map.
He subtextually let me know that he didn't want to play.