It’s a shorthand way of using a template into which variables are subsituted in a string. This allows for better reuse of string structures, and a few other goodies besides (like all V8 additions, it’s more than just a syntactical spruce up)
Bruce Mcpherson has a very useful post highlighting the benefits of using the new V8 runtime when working with string output
Source: Apps Script V8: Template literals – Desktop liberation
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.