Can’t sve

Hello I purchased because it was recommended online as the best ipad video editing software. 
i have stitched lots of clips together into a 20 minute video, done the transitions and added music  but can't seem to save it. It says the total file is 6.6 gb and myipad has over 100gb of storage left, yet Rush tells me to file is too big to export to my camera roll. It also told me it is too big to export to you tube. My hubby tells me it will also need 'rendering' but I don't know what that means, and a youtube search for rendering has been fruitless. Please Can someone help?