The first line means:
total: Your total (physical) RAM (excluding a small bit that the kernel permanently reserves for itself at startup); that's why it shows ca. 11.7 GiB , and not 12 GiB, which you probably have.used: memory in use by the OS.free: memory not in use.
total = used + free
shared/buffers/cached: This shows memory usage for specific purposes, these values are included in the value forused.
The second line gives first line values adjusted. It gives the original value for used minus the sum buffers+cached and the original value for free plus the sum buffers+cached, hence its title. These new values are often more meaningful than those of first line.
The last line (Swap:) gives information about swap space usage (i.e. memory contents that have been temporarily moved to disk).
To actually understand what the numbers mean, you need a bit of background about the virtual memory (VM) subsystem in Linux. Just a short version: Linux (like most modern OS) will always try to use free RAM for caching stuff, so Mem: free will almost always be very low. Therefore the line -/+ buffers/cache: is shown, because it shows how much memory is free when ignoring caches; caches will be freed automatically if memory gets scarce, so they do not really matter.
A Linux system is really low on memory if the free value in -/+ buffers/cache: line gets low.
For more details about the meaning of the numbers, see e.g. the questions:
- In Linux, what is the difference between "buffers" and "cache" reported by the free command?
- Why does Red Hat Linux report less free memory on the system than is actually available?
