Inertial forces are also known as G-forces. These forces are a result of a change in velocity or a change in direction. You experience G-forces every time you ride in a car. When you go around a corner fast and you feel like your body wants to slide across the seat, that's G-force. When you are pressed back in your seat by acceleration, or pressed forward under braking, those are G-forces.
Typically in a car we start to get uncomfortable when G forces exceed a half a G. That is, if I go around a corner fast enough to push me to the side with a force of half my weight, that's a half a G. That feels like I am going to slide out the door, and if I'm not driving, chances are my knuckles are getting white from holding on! Most street cars cannot generate more than about 7/10ths of a G; if you've ever been in a car being pushed to its limit, you know that this can be quite a dramatic experience!
Because it is wings, not tires, which produce the forces which airplanes use to change direction, they mostly produce G-forces in the vertical plane; i.e. up and down. When I first started flying aerobatics, we "pulled" 4 G's positive when we did loops. This meant that (since I weigh about 150 pounds) I was being pressed down in the seat with a force of 600 pounds. This was quite disconcerting! It felt like my face was being pulled right down off of my skull! It took me a little while to get used to this.
When we "went negative", meaning that while flying inverted, we pushed upward so that the G-forces were pushing me out of my seat, we saw over 3 G's negative on the G-meter. This meant that I was being pulled out of my seat, against my aerobatic harness (seat and shoulder belts), with a force of over 450 pounds. My arms tried to fall upwards off the stick, my hair went straight up towards the ceiling, and my headset shot straight up off my head! I had to get a cloth helmet just to hold my headset on.
Having flown only 5 G's positive and about 3 1/2 G's negative, I find it difficult to imagine what Unlimited Category competitors experience, when they fly close to 10 G's in each direction!