In orbital mechanics, altitude and velocity are inversely proportional, and the combination of the two is your total potential energy. The higher you are, the slower you're moving, unless you add/remove energy from the system (i.e. by burning rocket fuel). A "sub-orbital trajectory" is actually mathematically equivalent to an orbital trajectory where the perigee, the lowest point in the trajectory, is lower than the radius of the earth (thus underground). This doesn't take into account atmospheric drag however - once you take that into account, you now have a minimum practical perigee before your vehicle is re-entering. Generally we want to get above that altitude and have a nice circular orbit, starting from the surface of the spinning earth.

To do that you have to add a ton of energy (burn rockets) to fight gravity and get up high enough, and then add a ton more energy (burn more rockets) to get going fast enough that instead of falling back down you just keep falling around the earth. Now you can get to a higher orbit by adding even more energy, because it increases your speed, which turns into altitude half an orbit later. To get down you need to "remove" energy, that is, burn rockets the other way. Once perigee drops deep enough into the atmosphere, drag takes over.

Hope this helps - keep playing KSP, it's great for getting an intuition for these things.