The independent variable is the ONE factor the scientist/experimenter varies or changes during an experiment.
I see that the word ONE is capitalized in your question. This is probably because it's important that there's only one independent variable being tested in an experiment so that other factors aren't affecting the outcome.
There are three types of variables in an experiment:
The independent variable is what's being tested. For example, if you wanted to test the affect of particular fertilizer on plant growth, the fertilizer would be the independent variable.
The dependent variable is what changes in response to manipulating the independent variable. In the plant fertilizer example it would be the size of the plants.The dependent variable is what's observed, measured and recorded.
The controlled variables, also called controls, are the factors that the researcher keeps the same to ensure that there's only one independent variable. In the plant example the controlled variables would likely be amount of light, amount of water, type of soil, size of pot and variety of plant. The only difference between the control plants and the experimental plants is the independent variable.