A stellium is a multiple conjunction of planets. It's a close cluster of three or more planets in one sign and/or house. The planets are clumped together (within 0 to 5 degrees). This puts an energetic emphasis in the sign and house where it is concentrated.
This is something that stands out, when looking at a chart. A stellium in a birth chart is a special point of reference, and indicates the major themes of the chart.