There are few in features in my birth chart I love more than my Capricorn stellium — the fact that the sun, Mercury, Uranus, and Neptune were all arranged in Capricorn the moment I was born. In my mind, it's the astrological explanation for the way I communicate (awkwardly yet bluntly), how I confront change (poorly), and how I operate on a daily basis (steadily if not, again, awkwardly). And, it's why I come off so clearly and immediately as a Capricorn — far more so than if it was only my sun sign.
Depending on which astrologer you ask, a stellium must consist of at least three or four planets for it to be categorized as such. Either way, it simply refers to a concentration of planetary energy within any one Zodiac sign in your birth chart. Even if the stellium appears in a sign other than your sun sign (the sign you read your horoscope for), it will make that sign's traits more prominent in your personality and life. It's not the one and only sign you express, of course, but it might be more dominant than another sign that only plays host to one planet.
Obviously, the extent to which you feel the effects of your stellium depends on which celestial bodies are in it. If it mostly consists of outer planets (anything past the asteroid belt), its influence on you may be on the subtler side, since, due to their distance from Earth, these planets have less immediate effects on us. On the other hand, having a stellium that mainly consists of inner planets (like the moon, Mercury, Venus, or Mars) means expressing it more clearly.
For our purposes here, we'll focus on the effect a stellium can have when it's primarily made up of inner planets — with maybe one outer planet thrown in there — just to give you a better idea of the role one can play in your daily behavior and actions. Not sure if you have a stellium at all? Find out here (you'll need to know your birth date, time, and place).