This post explains output of free command in Ubuntu 16.04 paying special attention to information missing in documentation.
Let's look at free output in humanreadable, wide mode:
free -hw total used free shared buffers cache available Mem: 2.0G 566M 860M 14M 41M 531M 1.2G Swap: 4.0G 0B 4.0G
Note that output of free command in Ubuntu 16.04 differs from previous versions.
There are two lines in the output.
Mem row shows phisical memory stats.
Swap line displays swap stats.
total: Total installed memory.
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used: Cacluclated as total - free -