Some of these do have tooltips, so make sure to check by hovering your mouse cursor over the option in the launcher. Some don't, though, but it's always worth checking!
1) Fast Startup skips opening videos automatically, so you load straight into the title screen.
2) Clean Capture ensures no add-ons or (I think) external shaders show up in screenshots taken with the built-in screenshot function.
3) Bank Shortcuts allows you to use the number keys to jump through your bank list in different number increments. Try it out, see how you like it! It can be very helpful.
4) When you get hit, you have an amount of time where you are invulnerable. This time is measured in frames, with PSO running at 30 frames per second. I-Frames, or "invulnerable frames," are those frames that you are invulnerable during. One Person I-frames makes it so that the amount of i-frames you have after being hit in One Person Mode matches the amount granted when hit in previous versions of PSO when playing offline. This I believe results in more i-frames, meaning more time to escape or retaliate after taking a hit. Very noticeable in Ultimate mode, especially since outside of Offline/One-Person Mode, you don't get any at all (unless you get knocked down).
5) Menu Scroll Repeat Delay affects how long you need to hold up or down before the game starts automatically scrolling. Menu Scroll Repeat Interval affects how quickly it scrolls, by changing how long the game waits before moving to the next menu option. These are both measured in frames. Once more, PSO runs at 30 frames per second, so a Delay of 6 frames means you have to hold up/down for a fifth of a second before it starts automatically scrolling, and an Interval of 5 frames means it'll roughly go through six items a second while scrolling. Fun fact: you can scroll through menus with multiple directional inputs at once (example: you can scroll through your bank with left stick, right stick, and d-pad at the same time), and each input will have it's own Interval and Delay timers - meaning you can scroll through your bank significantly faster by holding up/down in multiple ways at once.
6) Page Up and Page Down Behavior affects how much hitting those keys on your keyboard scrolls through menus. For example, your bank. By default, hitting those keys will scroll a number of menu slots that is dependent on your resolution. This can feel very inconsistent, so you have the option to set it to Fixed, which allows you to set the exact number of slots hitting those keys scrolls by. Alternatively, you can set it to Auto, which always scrolls by a full page's worth of menu slots, regardless of what your resolution is and how many slots that involves scrolling through. I prefer it set to Auto, myself.
Bonus Question's Answer: Whenever highlighting a menu option in the lobby, you have to hit Tab on your keyboard to view the description for that individual option (if you're using a controller, you'll want to hit the left face button - Square on a PlayStation controller, X on an Xbox controller, etc). It won't change automatically, so you'll need to hit it whenever you want to see info for a different menu item. Hitting it multiple times will display further info in some situations. For example, you can hit Tab on a Party in the Party List to see the names, classes, levels, and locations of everyone in the party. Hitting it again changes it to show Game Type, the Section ID being used to determine drops, the Drop Style, if EXP Share is on, what quest they're on, and how long the party has been active.
