Total = is all the RAM you have
Cached = is the part where previously used programs or data are stored for a possible reuse (loads faster than from disk). That can be reused any time by active programs/data when needed. It also contains the so called Modified (see below)
Available = is the sum of cached and free
Free = is what it says. Nobody is using it .
You get a better picture of your RAM usage if you look into the Resource Monitor > Memory tab. The colored picture splits it nicely (see picture below). Everything in blue (light or dark) is available, There you also see 3 additional categories:
Hardware Reserved = is RAM that is needed by the hardware - e.g. for an on-board graphics
In Use = is what the system and data currently uses
Modified = is some temporary storage area where the system did not get a chance yet to write the data back to disk.