For the ‘Memory - Basic’ graph, the Free amount is entirely un-allocated memory, while the Available amount is the kernel’s estimate of additional memory currently allocated to temporary buffers, filesystem caches and so on, that’s still usable (‘reclaimable’) by the app if needed.
See the kernel documentation:
MemAvailable: An estimate of how much memory is available for starting new
applications, without swapping. Calculated from MemFree,
SReclaimable, the size of the file LRU lists, and the low
watermarks in each zone.
The estimate takes into account that the system needs some
page cache to function well, and that not all reclaimable
slab will be reclaimable, due to items being in use. The
impact of those factors will vary from system to system.
The ‘Available’ amount on the graph is actually MemAvailable - MemFree because the ‘Free’ amount is already displayed separately.