uptime

uptime prints one line: the current time, how long the system has been up, the number of logged-in users, and the load average for the last 1, 5, and 15 minutes.

$ uptime

 14:32:07 up 12 days,  4:17,  3 users,  load average: 0.42, 0.55, 0.61

Load average is the average number of processes either running or waiting for resources. As a rule of thumb, a sustained load above the number of CPU cores means the system is over-committed; values well below mean it has headroom. Uptime also appears in the top line of top, htop, and w.