Quick Text is a standard Salesforce Service Cloud feature that allows agents to insert custom predefined messages into emails. These are usually small messages that lack any graphical elements. You can read the SF KB article on setting up and using Quick Text here.
Provided that Quick Text is enabled, users see a new icon at the bottom of the email composer. Click on the Quick Text icon to open a new window where you can search and select a Quick Text.
As soon as you select a Quick Text, it is applied to your message at the location your cursor is placed. You can compose a message using several different excerpts of text and edit it freely after you add each one. Once finished, you can send your message for translation by clicking the Send button.