Yea, rtfm like the guy above said. But also:
total used free shared buffers cached
Mem: 3886 1720 2166 0 146 759
-/+ buffers/cache: 814 3071
Swap: 7632 0 7632
Those are the ones that matter, really. -m puts the values in megabytes. Linux caches stuff with free memory space and if you run out of memory for important stuff the caches get flushed. The man page doesn't really explain that.