An “en dash” is a hypen the width of an n while an “em dash” is a hypen the width of an m. Each has its usage in the English grammar which you can wiki to find out more. If you’re using two continuous hypens (–) in your writing, you probably meant to key in em dash but has no idea how to.
To key in en dash or em dash in your iPhone or iPad, press and hold the hypen key and then select the dash from the pop up.
For OS X, to key in en dash (–), press Option together with the hypen (-) key. For em dash (—), press Option together with Shift and hypen (-) keys. Please note that not all OS X apps and fonts support these two characters.