type desc
total physical mem
used used = total- free-buffers-cache
free Free/Unused memory
shared It is here only for backward comopatibility
buff/cache The combined memory used by the kernel buffers and page cache ans slabs. This memory can be reclaimed at any time if needed by the applicaitons. If you want buffers and cache to displayed in two separate columns, use -w options.
available An estimate of the amount of memory that is availablew for starting new applications, without swaping.

https://linuxize.com/post/free-command-in-linux/

https://www.cnblogs.com/peida/archive/2012/12/25/2831814.html

https://www.jianshu.com/p/2ffeb3a3aa90

https://www.codenong.com/jsafe796ceec93/