Space situational awareness (SSA) is the tracking of satellites and debris with sensors on the ground and in space, and the warning it gives operators when to move out of the way of a collision. Debris means old rocket parts and broken satellites.
Thousands of satellites and many pieces of debris now circle the Earth. Most orbit in the crowded region close to Earth. See LEO and GEO satellites. They move at several kilometres every second, so two objects that meet can destroy each other and create thousands of new pieces. A satellite operator needs to know where everything is.
SSA companies measure the position of each object with sensors. Radars and telescopes on the ground cannot see everything all the time. Telescopes need a clear, dark sky, and the ground radars see only objects that pass overhead. So some companies put sensors on satellites, to watch other objects from orbit, in any weather and at any time of day. The measurements are used to compute each object's orbit and to predict where it will be days from now.
Say a company operates ten small imaging satellites. The SSA service tells the operator that a piece of rocket debris will pass within 300 metres of one satellite in four days. The operator decides whether to move. The planning of the dodge is done by satellite collision avoidance software.
The operators who use this service include those flying Earth observation satellites and those who sell direct-to-device satellite connections to ordinary phones. They pay a subscription for the tracking and the warnings. Governments buy it too, to protect their own satellites and to watch other countries' satellites. As more satellites are launched, the risk grows, so the service is becoming a basic need.