Radial In
Radial in is basically anti normal, but horizontally in your orbit. Radial in is like, pointing inside your orbit. Think of a closed line, a good example would be in string theory. The craft is on the line, and points in the center of it, that’s pretty much radial in. But how do you use it and what it it good for? Well radial in is (in my opinion from what I’ve experienced) useful for changing your orbit’s eccentricity while anti normal changes the inclination, it can also largely change the arg of periapsis when you do a radial in burn at the right time, so be careful.
Radial Out
Radial out is pretty much radial in, but it is inverted. In Kerbal Space Program there is a tutorial that states “radial out basically points you up”, but that isn’t necessarily true unless you’re in surface mode. But for SimpleRockets 2 in surface mode there is literally a button for pointing up. Radial out is very handy when changing your orbit when your about to crash, but remember to point prograde when you attempt to avoid entering the atmosphere to do an unplanned aero break or something.
For working in circular or low eccentricity orbits, radial in/out does not get much use. It is useful when trying to adjust an intercept with a target closer while in similar orbits.
However, radial in/out is important for working with hyperbolic trajectories, and beyond, like brachistochrones. In other words, at relatively high velocities, it is most effective to burn radial in/out to adjust your course (adjust your closest approach to an object) rather than prograde or retrograde.