DESCRIPTION free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the kernel. The information is gathered by parsing /proc/meminfo. The displayed columns are:
total Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
used Used memory (calculated as total - free - buffers - cache)
free Unused memory (MemFree and SwapFree in /proc/meminfo)
shared Memory used (mostly) by tmpfs (Shmem in /proc/meminfo)
buffers Memory used by kernel buffers (Buffers in /proc/meminfo)
cache Memory used by the page cache and slabs (Cached and SReclaimable in /proc/meminfo)
buff/cache Sum of buffers and cache
available Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items being in use (MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)
Buffers %lu Relatively temporary storage for raw disk blocks that shouldn't get tremendously large (20MB or so).
Cached %lu In-memory cache for files read from the disk (the page cache). Doesn't include SwapCached. SReclaimable %lu (since Linux 2.6.19) Part of Slab, that might be reclaimed, such as caches.
SUnreclaim %lu (since Linux 2.6.19) Part of Slab, that cannot be reclaimed on memory pressure.