A responding variable is something measured in a scientific experiment when another variable (called an independent variable) is changed in a controlled way.
An example is sunlight and plant growth. You can measure the growth of a plant as affected by how much sunlight it receives. You change the amount of sunlight the plant is exposed to and measure the growth of the plant against the amount of light.
Another example might be the affect of temperature on the growth of bacteria. Change the temperature and observe the growth.
In the case of the plant and the bacteria, the growth is the responding variable.