This means that two characters or words need to be switched in order to correct the text.
For example:
* "teh" would be marked as "ts" to indicate that the "h" and "e" should be swapped to read "the."
It's important to note that "ts" might have other meanings depending on the context. For example, it could be used as an abbreviation for "typescript" or "times."
If you are unsure about the meaning of a proofreading mark, it's best to ask the person who made the mark.