The uptime command shows the length of time since the system booted up. The output is displayed on the standard output.
break# uptime 2:49PM up 229 days, 30 mins, 2 users, load averages: 0.00, 0.00, 0.00
The above shows that, at 2:49pm (when the command was run), the system in question had been running for 239 days and 30 minutes. It also had two users logged on. The three load averages values show the system utilisation as at 1, 5 and 15 minutes ago respectively.
Note: FreeBSD does not have an "uptime reset" issue, often referred to as the 497 day roll-over bug, that most Linux and a few UNIX and Unix-like systems do. (Note: to determine genuine uptime on linux systems affected by the roll-over bug, the following command may be of use: last -xf /var/run/utmp runlevel.)
There is a project on the internet called Netcraft that monitors uptime of websites. It uses the TCP/IP counter to measure uptime and as such the values are not always accurate. That said, the FreeBSD (and other BSDs) are frequently topping the chart with regards to total uptime.
See also w command.