You can learn more about working with Notes in Redtail here. In this post I just want to cover how you would go about adding the same Note for multiple contacts at one time.
From your Advanced Search page, after running a search that will include your desired contacts, place checks to the left of the names of those contacts to whom you want to add the Note:
In the example above, I've run a search that yielded 99 records (only partial results are displayed in the screenshot) — I then selected six of those records for whom I want to add my Note.
My next step would be to click Options in the Result Options widget to the right of my search results and then to select Bulk Note Entry from that menu:
After clicking Bulk Note Entry, I'll then see the Add Bulk Note Entry dialog, where I can add my Note, choose a Master Category for the Note and then click Save Note:
After saving the Note, each of these six contacts will have whatever was entered in the Note field recorded as a note within their record.
As you can probably see, it's a quick and easy method to add notes to multiple records at one time, when the need arises to do so.