Courtney - You've stepped into the Compression Vortex! :-)
Simplest answer is that a given level (3.0, 3.1, 4.0, 4.1, etc) and profile relate to how high of a data rate and frame size you can encode. I won't enumerate all of them here, but Wikipedia has it all written down for you:
https://en.wikipedia.org/wiki/H.264/MPEG-4_AVC#Levels
Ultimately, it depends on your audience that you're trying to reach and at what resolution/data rate as to how you encode. So if you plan on delivering 4K files at the max data possible to your end user(s) you'll have to use Level 5.1 or 5.2 in High Profile (300MB/s). But if you're going for the widest possible audience you'll have to take into account people who have older devices that may not be able to read that high of a level. For example, people with old Samsung Galaxy phone or an iPhone 4 or 4s. In those cases you need to back your level off to something in the 4's. For my encoding when delivering to 720p for the web I go back to level 3.1 or 3.2 to ensure the widest possible compatibility range for devices.
FYI, the current iPhone 6 only reads up to level 4.2 (
https://www.apple.com/iphone-6/specs/ ):
Video formats supported: H.264 video up to 1080p, 60 frames per second, High Profile level 4.2 with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats.
Hope that makes some sense for you....!
Ryan Holmes
http://www.ryanholmes.me
@CutColorPost
