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:
Then, click the Contact Options menu above your search results and select Bulk Actions:
Next, select Note Entry from the Bulk Contact Actions modal that appears.
After clicking Note Entry, you'll then see the Assign Note modal:
You can then enter your Note along with any formatting and click Save Note. Each of the 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.