In later pictures, amber dots show current, the flow of electricity. Here the blue dots are electrons.
Each red plus stands for a missing electron. That leaves a proton with no electron to balance it. A ball with fewer electrons than protons is positive.
Every ball also holds a huge number of electrons and protons that balance out. The marks show only the extra or missing ones. A real charged ball has billions of them, so each mark stands for a great many.
Both balls feel the same force, in opposite directions. The arrow shows the force on the hanging ball. Its length shows the force only roughly. The shortest arrow is drawn a little longer so you can see it.
At half the distance, the force is four times as strong, because 2 × 2 = 4. At twice the distance, it is a quarter as strong. Each number compares the force with the force at 6 cm.
The stand is moved so the centers of the balls stay at the distance you pick.
This rule is exact for tiny points of charge. For real balls this near each other, the rule is close but not exact.
The hanging ball leans until the force, its weight, and the pull of the thread balance. The amount of charge is picked so the lean is easy to see.
The thread is silk and the stand is plastic. Electrons barely move through them, so the balls hold their charge for a while.
In a wire, electrons move from the negative end toward the positive end. Later pictures draw current the other way, from positive to negative. Most diagrams draw it that way.
An AA battery is rated 1.5 volts. Voltage is always measured between two points. Here the two points are the battery's two ends.