The 1 is the number of times the command is to be issued.
In this case, once. If you used a 0, then the command would be issued indefinetely.
The 5, in case you're wondering, is the number of seconds delay between the time the /timer command is issued, and the time the command in the /timer (in this case /join) is issued.
The help file is half-decent at explaining this, and other options that are available. See /help /timer