To understand that like charges repel and unlike charges attract each other.
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.
In physics, fields describe the distribution of forces in space. Their description thus requires the knowledge of magnitude and direction. Both are expressed through field contours; field forces act in the direction of the contours, and the distance between the contours indicates their magnitude. (The closer the contours are together, the larger the force.)
In an electric field the direction of the field is given by the electric charge of the charged sources. If the charge is negative, the field is directed toward the charge. All electric fields begin on a positive charge and end on a negative charge.
If two positive charges interact, their forces are directed against each other. This creates a repellent force. (The same occurs with two negative charges, because their respective forces also act in opposite directions.)
When the electric field and resulting forces produced by two electrical charges are of the same polarity, the two charges repel each other.
If a positive charge and a negative charge interact, their forces act in the same direction, from the positive to the negative charge. As a result opposite charges attract each other:
When the electric field and resulting forces produced by two electrical charges are of opposite polarity, the two charges attract each other.