Let's take on a concept that is one of the harder ones to wrap your mind around; namely, a forks "trail". Get a cuppa joe, you need to be awake for this! I'm not talking about a dirt trail, I'm talking about fork trail!
The concept of trail for a bicycle is a little tough to grasp at first, but this example lead me to the light. Let's see if it does the same for you. Okay? Think of a shopping cart wheel, or a caster on an office chair or dolly. Got that? Okay, think about what happens to that caster wheel when you push the cart/ chair/ dolly. The wheel pivots around so it "trails" the axis that it pivots on, right? Ta da! You've got trail, mate!
Your bicycle works in the same way as the caster wheel. Push your bike forwards by the saddle. The wheel wants to wobble a little, but you can go forwards without much trouble. Now try the same thing backwards. Wham! Handle bar slams top tube! This is because the bicycles front tire is trying to do what a shopping cart wheel will do. It's trying to turn negative trail into positive trail. Or in other words, it's trying to get the contact patch of the tire to the ground behind the axis which the fork spins on. This is the self centering effect that allows you to ride no-handed. If the bicycles fork geometry had no trail, or negative trail, you would find that riding the bike would be impossible, extremely difficult, and certainly not possible to do no-handed.
Well, I hope that clears up what a forks trail is and how it makes your riding experience a good one. Next up: a little math on fork trail, offset, and head angle! Class dismissed!