Subtext is the message you are getting without it actually being said. It can be applied in dialogue, detail, and even in foreshadowing. The point is to add depth to your story.
For example, if two street fighters met up in an alley and one of them asked the other to “dance”, you would understand that this doesn’t really mean he plans to waltz and the intention is to fight.
If you’re using subtext in detail, you would know that saying something like “the crooked man struggled to reach the top of the staircase” has a far different effect than “the spry man bounded up the steps two at a time”. We didn’t state whether the man was old or young in each case, but from our descriptive, we were able to determine that.