<cfif Len(Trim(Form.Comments)) GT 0>'#Trim(Form.Comments)#'<cfelse>NULL</cfif>

Without the context of the code is impossible to know what exactly that code does, but for example, if you are inserting or updating a field in a DB, the situation could be (inside a CFQUERY):

UPDATE table SET field=<cfif Len(Trim(Form.Comments)) GT 0>'#Trim(Form.Comments)#'<cfelse>NULL</cfif>

The code means : If Form.Comments is not empty (len > 0) then return Form.comments trimmed, else return NULL.

Regards