In the dictionary, it is interpreted as "the implication between the lines or not fully expressed between the lines", which is "there is something in the words" in a popular saying.

Don't underestimate the role of subtext. Actors not only make full use of it to enrich the expression of lines in film and television dramas, but also benefit you a lot in many places.

For actors:

Subtext is the true inner expression of characters in the process of action, and it is the soul of expressing characters' images. It is a key to mastering lines. The same lines and aria will be mastered differently because of different understanding of subtext. Good at digging subtext. When the subtext is dug out, the action of the characters comes out. If we find the subtext, we will find the true thoughts and feelings of the characters and shape their expressions, actions, languages and so on. There is also a basis.