Anything short-term doesn't last long. A short-term romance might be for a few weeks instead of a lifetime, and a short-term job won't provide you long-term security.
The word term often applies to units of time, like a politician's term in office. This word has to do with time, but only small amounts of time: anything short-term occurs for a little while, and then it's over. A short-term illness goes away quickly, and a short-term problem won't weigh you down for long. The opposite of short-term is long-term, which refers to things happening for a greater period of time.