A minimum spanning tree (MST) is a spanning tree where the sum of edge weights is as small as possible. If the graph has multiple spanning trees, the MST is the cheapest one. For example, if you are connecting cities with roads and each road has a construction cost, the MST gives you the cheapest way to connect all cities.
The MST is not always unique. If edges have equal weights, you might have multiple valid MSTs with the same total cost.