What does it mean that it can only be understood but cannot be expressed in words?

It can only mean but cannot be expressed in words.

People often say that it can only be understood but cannot be expressed in words.

One meaning. It’s (this matter) that you and I can only “understand”. That is: everyone "tacitly understands" this matter. It's inconvenient to say it, and it's hard to say it out loud (if it leaks out, the consequences will be bad and unimaginable).

Two corrupt officials accepted bribes at the same time, so this is the case.

You discovered the matter between a certain leader and the mistress. At this time, a certain leader also saw you, with the same attitude: you can only understand it, but cannot express it in words. ha.