MemTotal: all memory installed or provisioned to the system
MemFree: memory without anything in it (wasted)
SReclaimable: This is the part of the slab or “in-kernel data structures cache” that can be reclaimed.
ActiveFile: Amount of buffer or page cache memory that is in active use.
InactiveFile: Amount of buffer or page cache memory that are free and available.
WatermarkLow: Placeholder memory
MemAvailable: A more accurate estimate of how much memory is available. (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0a)