Selecting this option opens a small requester asking you for the text you wish to search for ,If you enter the search text you have two options `FIND` which will find,goto and highlight the search text if found,and then close the requester.
Or `FIND NEXT` which if you keep clicking on will search for more instances of the search string.
You also have a small check box which gives you a `REPLACE WITH` option,which if checked will replace the searched for and found text with the replace text entered in the requester.