The Hardware Information section lists your computer's CPUs and main memory.
Use ... CPUs: Controls how many CPUs (or cores, no distinction is made) the application uses.
all: use all CPUs
only: use a specific number
all but: use all bar a specific number (e.g. use all CPUs bar 1 to leave 1 core free to perform general system background tasks).
The number in brackets shows how many CPUs will currently be used and how many are available in total.