Confounding is an incredibly easy concept. In a study, you manipulate an IV in order to measure its effect on a DV.

For your experiment to be valid, the only factor operating should be your IV. However, this can go awry and some other process or factor is influencing your study. When this occurs, your study is confounded.

Confounding can occur when the experimenter accidentally manipulates the subjects in an
unintended fashion, the subjects are influenced by merely being in an experiment and this is mistaken for a treatment effect, the groups are selected such that there is a bias between groups, and many other possible screw-ups.

Appropriate procedures exist to test for and to control these effects such that your experiment will be valid.