The Canvas in Figma is the backdrop on which all of your frames, groups, and layers live.
By default, vectors are rendered as resolution-independent paths in the canvas though you can turn on Pixel Preview at 1x and 2x resolutions to enable pixel-precise editing for raster assets.
You can change the background color of the canvas in the Inspector by de-selecting any layers and clicking the color picker labeled “Background.”
If you like having Rulers visible on your canvas, you can use the Shift + R shortcut to enable them. You can also use the search feature under the menu icon to search for rulers.