A Spanning tree is a graph which has no cycles.
Ethernet cannot handle a loop, as it would mean that switches will forward packets around the loop continously. However, without loops you cannot have redundancy, so a protocol called SpanningTree was invented. Switches that participate in spanning tree shut down links that would cause a loop, until whatever was causing the loop "goes away". This is often used for redundancy.
One page links to SpanningTree:
