The ruler appears by default when you tap in text, but you can hide it if you want.
The ruler is visible when text is selected and hidden when an object is selected. If you don’t want the ruler to appear at all, you can turn it off.
Tap 
in the toolbar, then turn off Ruler.
If you want to align objects, you can turn on alignment guides.
Tip: You can also use the ruler to set margins and tab stops.