When editing or creating a communication, you would click on this icon to access your token library.
Here you can see a list of tokens that are available to you.
Here's how to understand which tokens you may want to insert into your communication. Recipient Tokens are used to fill in the information for the contact(s) you are sending the communication to. For example, {FNAME} fills in the information that is in the first name of your contact.
Your tokens fills in the information that is in your profile. For example, if you insert [firstname] it inserts the information that is in the first name field of your profile.