A digital transmitter does not need to send one physical pulse for every individual bit. It sends symbols, where each symbol is chosen from a finite alphabet of possible signal states.

If the alphabet contains

$$M=2^k$$

symbols, one symbol can represent $k$ bits.

For symbol rate $R_s$ symbols per second, the corresponding uncoded bit rate is

$$R_b=kR_s=R_s\log_2M.$$

The symbol interval is

$$T_s=\frac1{R_s}.$$

Bit rate measures information representation speed. Symbol rate measures how quickly the physical signaling state changes. They are equal only for binary signaling with one bit per symbol.

Increasing the number of bits per symbol can raise bit rate without raising symbol rate, but the receiver must then distinguish among more possible signal states. For a fixed available signal range, those alternatives generally become more closely spaced.