Shortcodes in SM insert custom information from your account into templates. An advantage of shortcodes is that you can change a piece of information once, i.e., your phone number, in the Control Panel, which is then altered across every template in your library. Never use hard data in your templates (like a phone number 336.555.1212) where a shortcode is available.
Shortcodes also insert variable information like invoice numbers and proposal links. They are powerful. You should copy and paste any shortcode from the Help of Service Minder, as capitalization and spelling are critical. Also, test any template used with shortcodes before sending it to a customer.
Rather than try to remember any of the 100's of Shortcodes, you can refer to this article in SM.