Uptime - shows average load and how long has the system been up

It is in ubuntu server distribution, no need to install, this command shows the current system time, system uptime or how long the system has been running, how many users are currently logged on, the system load averages for the past 1, 5, and 15 minutes, same information is displayed in the header, when you run w command.

$ uptime

Command output:

 08:13:22 up  2:06,  1 user,  load average: 0.91, 1.17, 1.23

System load averages is the average number of processes that are either in a runnable or uninterruptable state. A process in a runnable state is either using the CPU or waiting to use the CPU. A process in uninterruptable state is waiting for some I/O
access, eg waiting for disk. The averages are taken over the three time intervals. Load averages are not normalized for the number of CPUs in a system, so a load average of 1 means a single CPU system is loaded all the time while on a 4 CPU system it means it was idle 75% of the time. Source: man uptime

Load average explained

In above example, load average is 0.91, 1.17 and 1.23, it is a virtual machine running on Amazon AWS EC2, a m1.large instance with almost 8 GB RAM and two processor cores assigned, this load average is at Monday morning in a time when web application on this server has on average 200 visitors at a time, so load average 1.23 is OK, because as long as it is divided for two cores, it is similar to saying that CPU is used on 61.5% (1.23/2*100). For example, if we would have a same load average of 1.23 on one core processor machine over 15 minutes, that would not be so good, because that would be almost constant 123% (1.23/1*100) CPU overload. If you would have a i7 processor, it has actually only 4 real cores, it would be calculated as 1.23/4*100 = 30.75 CPU usage, that is very acceptable. 

Load average 1 on one core processor means, that your processor is being used on it's full potential and more long time load would overload it, so if you are planning to run a business application that needs to be up and running, try to accomplish load average less than 1.

Load average 1 on four cores processor means, that your processor is being used on 25 of it's full potential, depending on your application running on the server, it is acceptable

Beware, that it is only 1, 5 or 15 minutes average, you machine may have a lot of spikes over the longer period of time, for example when some faulty processes run, when running cron jobs, when more users are connected in short period of time or when server is attacked, or indexed by GoogleBot at night, you never know by using uptime or top commands, that is why it is better to rely on monitoring tools, that log CPU and memory activity in long term.