Manual: A manual is when you lift the front wheels of your skateboard up off of the ground, and balance on two wheels while moving. You can either do it while riding normally, or after ollieing up onto something (ex. curb/sidewalk).
To manual without ollieing onto something, do the same thing, but instead of ollieing and landing on the back wheels, simply gradually shift your weight to the back of the board while riding, and allow the front of the board to come up, keeping your front foot at the center of the board to balance yourself.