Plots is creative-mode server where the world is divided into plots. Players can claim plots in which they're allowed to build. Building is restricted outside of plots so this ensures everyone gets space to freely build without encroaching on other players.
To get started, you can automatically claim the nearest available plot with "/plot auto", or by using the Plots Menu with "/pmenu" and clicking "Claim a plot".
Alternatively, you can fly around and try to find an unclaimed plot in a place you like and claim that plot specifically by standing in it and using either "/plot claim" or the Plots Menu and clicking "Claim this plot".
Once you've claimed a plot, you can always get back to it with either "/plot home" or the Plots Menu.
To make building easier, some physics are disabled such as blocks breaking when the block under them is broken. Carpets, for example, can be placed on a block and then you can break the block under it and the carpet will remain in the air.
You can access blocks normally unobtainable through the Creative mode inventory with "/blocks".
You can also toggle helpful build features on/off with the command "/bu".
If you've built something you'd like to download as a .schematic file for pasting into your own worlds, you can click "Download the plot" in the Plots Menu while standing inside the plot. You'll be given a link to a web interface where you can download the plot.
You can add other players to your plot to give them the ability to build with "/plot add " or by using the Plots Menu. Similarly, you can remove them with "/plot remove ".
For a more detailed list of all the commands you can use, see this post