CPU, Memory, Storage, Network.
CPU
A modern CPU has multiple cores and it's own memory. These specs on the box help compare performance with other CPUs.
The system measures CPU resources in terms of usage per core, and usage over time, as the load.
Memory
8gb is a minimum for decent desktop performance, and 16gb is needed for running virtual machines. New machines and servers can run a lot more.
Since memory can swap to disk when not used, as long as you have enough for your current workload you won't notice any slowness.
Storage
Drives have capacity limits, and also read/write limitations that can become a bottleneck if heavily used by multiple applications at once. Servers have more advanced storage capabilities like multiple drives in a RAID configuration.
Network
Your network interfaces have rated max speeds, along with the network hardware, cables, and service provider.
Watch the activity charts to see which applications are using the network.