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I often use a column I call “explanation” and put the addition information (sometimes several paragraphs worth) in there. You could also and a link to another view in the app with more information or a link to web page.
Sometimes I’ll make use of an action, setting it to be inline with a column, that looks like a question mark; when people tap it, it’s just an alert action with some text in it.
You can easily create alert actions like this, where you don’t need anything to happen but show a little text, by using the Group action; if you don’t put any actions in the stack, but set the confirmation to happen, you get an action that creates a pop up. 
Now if you REALLY wanna get crazy 
, you can create an actual in-app help system - with a table for users to submit a help request, and a followup table for the answers.
You can go EVEN further and take this help system and integrate actions (like I was describing above) onto the fields that people were asking about with deeplinks taking you to the specific help request and followup.
The final way I’ve done this is to create similar fields like what Kirk is doing:
But I’ll include a UserSetting to turn these on/off. For people like me, the dev of the system, or other power users, we can skip those - they just get in the way. 
Here’s another example of how I add explanations to pages in my app. It’s part of a “feature request” for a hide / show toggle. We don’t have that yet, but, for now, this is another example of my approach. If you think it would be nice to have a hide / show toggle, please vote for it. 
