What is buffers/cache in free command in Linux ?

When we run free command to view the memory usage in Linux we get a similar output as below

# free -m
             total       used       free     shared    buffers     cached
Mem:          1869       1398        471          0         24        400
-/+ buffers/cache:        974        895
Swap:         3999          0       3999

Now here most of the terms should be understood but ever wondered what is that buffers/cache ?

Let me explain all the parameter used here

total: It shows you the total memory assigned to your machine but again the value shown to you is not the real value as it excludes a small amount of RAM which is reserved for kernel as in my case my machine has 2048 MB but out of which it only shows me 1896 MB where 152MB is reserved by kernel for booting purpose
used: The total used memory by the processes in your OS
free: Memory not in use

total (1869) = used (1398) + free (471)

Now coming to -/+ buffers/cache

First value i.e 974 in my case gives the original value of used minus the sum buffers + cached 

( 1398 - (400 + 24) ) = 974

Second value i.e. 895 gives the original value for free plus the sum buffers + cached 

( 471 + 24 + 400 = 895 ) = 895

Again the last section shows you the total assigned, used and free swap memory having no much of any mathematical calculation.

How do you understand if your system is low on memory?

This question can be a bit complicated to answer unless you understand the difference between Virtual memory, used memory and buffers/cache.
Most of the time you observe the output of free command, free memory section will be low value but comparatively buffers+cache value would be higher. Now this is not a bad thing actually since your OS has reserved this memory to speed up your most used process by keeping them in the cache. But in case any new process is executed and your system is low on memory then these cache would be automatically released to make space for memory reservation of new processes.

How to free cache memory?

You can do this using the below command

# echo 3 > /proc/sys/vm/drop_caches

These are the different values which you can use with the above command
echo 1 is clearing only page cache
echo 2 is to clear free dentries and inodes
echo 3 is clearing page cache, dentries and inodes