When viewing different items in the mobile, you can create new items. For example, when viewing an Opportunity, you can create a new Task. The new task will be pre-populated with the details from the Opportunity.
To create a new item while viewing an item, click the + icon located in the upper right corner.
This will pop up a menu of item types you can create. Tap the item and it will be created and be pre-populated with the details from the item you're viewing.
If you tap the Create New icon in the Toolbar at the bottom of the screen, this will create a new blank item with none of the fields pre-populated.
The ability to create new items and the type of items you can create will depend on the type of item you're viewing. Not all items have the ability to add a new related item. If you see the + icon in the upper right corner of the screen, then you know you can create a related item.