If you are reporting on a large text field, like an Activity or Case Description, then it is possible that you can have hundreds or thousands of characters in this field. You may not want to display all of these characters in a report, especially if you are sending the report in a scheduled email. There is a simple function, LEFT() that can help you to cut down the number of characters that are displayed, e.g.:

LEFT( description, 30)

will only show you the first 30 characters of the description.

If there are more than 30 characters, and you want to make it clear to users that there is more text beyond the 30 characters, you could add "..." to the end:

CONCAT(LEFT( description ,30), IF( LENGTH(description)>30, '...', ''))

This will make:

"A long description that is more than 30 characters"

become:

"A long description that is mor..."

But 

"A short description"

does not get shortened or have "..." added to the end as it is less than 30 characters.