Figure 1 shows the circuit diagram of differential amplifier with swamping resistor. The English meaning of Swamping is a lowland region which is saturated with water. Same convention applies in differential amplifier that is a resistor Re of order of $50\Omega to 100\Omega$ connected in the emitter terminal of both the transistor of a differential amplifier is known as swamping resistor. This resistor has following effect:

Gain of differential amplifier decreases.

Differential output of differential amplifier decreases. Since Vo=AdVd

Input resistance increases due to series negative feedback.

Stability of differential amplifier increases i.e. output of differential amplifier will be stable against temperature and $\beta$ variations.

Bandwidth increases.

Circuit becomes less susceptible to noise because of negative feedback.