A break is a special signal sent independently of the stream of bytes over a serial line.
You'll often see "CTRL" + "BREAK" referenced in documentation, this refers to the keyboard shortcut used in Terminal programs like PuTTY to send a break signal.
The RS-232 specification defines a break signal as when the data line remains in the space condition for a specified duration, usually at least 100ms. This state is distinguishable from normal data being sent over the line, and can trigger a specific action on the receiving end of the connection. A break can be used to enter SysRq mode on some Linux machines, and ROM Monitor mode on Cisco devices when a password recovery is necessary.
To send a break, go to Terminal → Send Break, or press ⌘ + b. This will hold the line in the break state for 250ms.
To send a long break, go to Terminal → Send Long Break, or press ⌘ + shift + b. This will hold the line in the break state for 3500ms and hold DTR low for the duration of the break.
NOTE: Some devices and/or drivers do not properly support breaks. In these cases Serial will emulate a break by momentarily dropping down to the lowest supported baud rate and sending a NUL character.