The start bit is nothing to do with the Modbus protocol specifically.

It is associated with ANY form of asynchronous serial communication (which Modbus RTU is an example of).

It forms part of the message character framing. The data line sits at the "idle" state. The line goes active for one bit period that marks the start of the next character, this is known the start bit. There is not option to change the amount of start bits, as it is always fixed at 1. There then follows a configurable number of data bits (7 or 8), then an optional parity bit, then a configurable number of stop bits (1 or 2), which mark the end of the message frame. After the stop bit, the data line returns to the "idle" state. Normally 1 stop bit is used. However, there was an option for 2 stop bits. The reason for 2 stop bits may surprise you. Back in the days of teletype machines, the extra stop it gave a bit more time for the mechanical bits in the teletype head to stop, before the next character came along.

Some Modbus RTU devices have the option to having
- 7 or 8 "data bit"
- 0 or 1 "parity bit"
- 1 or 2 "stop bit"

Modbus RTU is always 8 data bits, by the Modbus spec.

Modbus ASCII is always 7 data bits, by the Modbus spec.

Sometimes offering a serial configuration option of 7 data bits for Modbus RTU creates a problem, because 7 bits won't work with RTU.  

The Modbus spec says that "no parity" must use 2 stop bits, but very few, if any, implementations pay attention to that.  8-0-1 is very common and usually works OK.   In fact, the ability to choose 2 stop bits is missing/not available in many slave/server devices.