The first factor is concentration. Now we're going to say, in order for a reaction to occur, molecules must collide. So molecule a has to combine with molecule b. Now, what we should realize here is increasing the number of molecules in a container, we're going to have more of them bouncing around, so there's a greater chance of them colliding. Hence, adding more molecules inside the container increases their collisions and as a result we have a greater chance of them sticking together. Them sticking together to form our product is the reaction.
We're also going to say they not only need to collide with each other, but they need to collide with each other with sufficient energy, and they have to hit each other in the correct spots. So, they have to hit with sufficient energy and correct orientation. In terms of sufficient energy, just think of it as this: you have two cars going head to head against each other. If they're going at 5 miles per hour, they're gonna hit each other but they're not gonna be able to stick together. But let's say both cars are both going at 100 miles per hour towards one another, that head-on collision will force them to be basically smashed together. In the same way, our molecules do the same thing. Molecules have to be moving incredibly fast in order to stick to one another. Otherwise, their collision is going to be elastic, meaning they'll just bounce off each other.
Also, if you guys are biology majors, just remember, we also talk about activation sites. Basically, they have to hit each other in the correct spot, and in that way, they'll stick together. Again, if they don't hit each other in the correct spot, they're just going to bounce off of one another.