SELCAL stands for Selective Call. It is a technology used during trans-oceanic flights that eliminates the need for pilots to monitor noisy HF radios continuously. Each aircraft has a unique four letter SELCAL code. Controllers wishing to get the attention of a particular plane can send a coded sound representing that plane's SELCAL code over the active frequency. This will cause an alert to display in the cockpit.
SquawkBox simulates this by allowing you to set a SELCAL code as a property of aircraft in your aircraft list. If a text message is received on the active frequency of the form "SELCAL <code>" the tones associated with the indicated code are played. If the code matches your current aircraft's SELCAL code, a warning sound will also play and a warning message will display in the main SquawkBox window.
You can disable this functionality altogether in the Advanced Options.