The baud rate is the rate at which information is transferred in a communication channel. Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in bauds. Ref: https://en.wikipedia.org/wiki/Serial_port#Speed.
"2400 baud" means that the serial port is capable of transferring a maximum of 2400 bits per second."
So, transmission rate here = 2400 bps
An eight bit data (which is a char) requires 1 start bit, 2 stop bits = 11 bits.
So, number of characters transmitted per second = 2400 / 11 = 218.18.
Since we can only count the fully transmitted ones, we take floor = 218.