If you have a negative electron, then it will attract a positive charge.
- Put that positive charge above the electron, and it will be attracted downwards.
- Put it below the electron, and it will be attracted upwards.
- Put it besides the electron, and it will be attracted sideways.
Each of these paths that the positive charge would want to move along, is called a field line. Clearly, the electric force direction (the electric field lines) varies with location in the vicinity of the electron. But also the magnitude varies:
- A positive charge put above the electron is attracted downwards.
- A positive charge put above, and a bit further away, is attracted less strongly downwards.
- A positive charge put above, but a bit closer, is attracted more strongly downwards.
When further away, the field lines are farther from each other than closer to the electron. So basically, if you draw many field lines, how closely spaced they are tells us where the electron attracts more strongly. See for example this graphic from Wikipedia:
Closer to an electron, the field lines are closely spaced. But with two electrons put together (left illustration), the paths a positive charge would follow are distorted because it now is attracted towards two points simultaneously.
Or equivalently (right illustration), imagine having an electron and a positive proton in the same vicinity. The electron attracts a positive charge, whereas the proton repels a positive charge - so at a point in between them, a positive charge is both pulled in and pushed in, giving a stronger combined pull and thus closer field lines. The combination of field lines at each point as shown gives us a visual idea of where the attraction is largest and where it is smaller.
All such field lines together (if you imagine drawing up infinitely many of them) make up the electric field.