The word kundalini refers to an energy that everyone has within them, but that usually lies dormant. What this means is that the majority of people don't feel this energy and, much of the time, don't even know it exists. However, in a few rare cases, this energy can come to life, getting activated and aroused. Depending on the person and their knowledge, readiness, and viewpoint, this can be a joyous occasion or something that disturbs and frightens them a bit. People who intentionally awaken their kunalini will have a positive reaction to this phenomenon.
The word kundalini is a Sanskrit word which translates to the word "coiled", similar to the posture a snake adopts when it lies on the ground. This energy has not been recognized or acknowledged by science in the medical field and even among meditative experts and longtime teachers. The definition is rarely ever fully understood. However, this phenomenon is thoroughly discussed and mentioned in important texts about tantra and yoga, including both Hindu and Buddhist sources.
Since this word has been used so widely, a lot of people think of it as the material of legends or simply a useful metaphor for personal growth or transformation. Some have even defined it as a person's creative potential. While this may be the case for some, who can look to symbolism to gain meaning in kundalini, it's a very real and quite literal experience for others.