Free memory
This memory is not being used currently
Wired memory
This information can't be moved to disk, so it must stay in RAM. The amount depends on the applications you are using.
Active memory
This information is currently in RAM and has recently been used.
Inactive memory
This information has not recently been used but will remain in RAM until another application needs more memory but no free memory is available. If called upon by a process, this is quickly changed to Active memory; if it has been swapped to the hard disk, it will be moved back to RAM and marked as Active.
Linha de fundo: se mais memória for necessária, você ainda tem 'Blue' na reserva.