Reflowing refers to reconstructing lines broken by line wrapping. Here's the example I was referring to: Grab a couple of unwrapped paragraphs of lorem ipsum (make sure there's no explicit newlines) and email them to yourself as a plain-text email using the desktop web gmail interface. You will notice that when you open the email in your inbox, it shows up with line breaks - Gmail sent it with explicit newlines after ~80 characters. (Use Show Original if you want to confirm.)

Now open the same email in the Gmail app on Android (I'm using Gmail 2.3.6 - the newest available on my Androids 2.2 and 2.3.6). My Android screen, like most smartphone screens, isn't wide enough to hold all 80 characters in one line, it's more like 50 or 60 characters. Gmail wraps the email to fit, which is expected and desirable. The problem is it keeps the original linebreaks at 80 characters, resulting in two linebreaks. Every other line is short at 20 or 30 characters and the email looks very jagged. Reading it is very uncomfortable. It gets really messy when you have quoted text, particularly with interleaved response style.

There's actually a whole bunch of standards or semi-standards and techniques around doing this kinda stuff. Standalone mailers have been doing reflowing and format=flowed for a while now. Sad to see Gmail drop the ball on this.