What does isothermal expansion mean?

Hint: There is no change in temperature in an isothermal process, which means the temperature remains constant. When hot water is stored in a thermos flask and a particular amount of water is removed while keeping the temperature constant, the process is called an isothermal process.

Complete answer:
Any process in which the system's temperature remains constant is known as an isothermal process. The change in internal energy of a closed system is equal to the heat transferred into the system plus the work done by the system, according to the First Law of Thermodynamics. Expansion is the process of increasing the volume of a thermodynamic system. As a result, when an ideal gas expands isothermally, the temperature remains constant as the volume increases.
Isothermal expansion occurs when a gas expands from its initial volume to its final volume while maintaining a constant temperature. Only in the case of an ideal gas does the internal energy and enthalpy depend on the temperature.
As a result, the amount of heat transmitted into the system is equal to the amount of work done on it by the environment. When we plot an isothermal process on the $xy$ plane, we can see that as pressure rises, the volume falls, and as pressure falls, the volume rises.

Note:
 Internal energy is a function of temperature and pressure, and the volume product is also a function of temperature, therefore in the case of an ideal gas, enthalpy is a function of temperature, and the change in enthalpy is also zero because the temperature is constant. As a result, enthalpy is unaffected by the isothermal expansion process.