What causes spring and neap tides?
The moon creates the normal tides, but the sun may work with the moon or against it.
Let's first forget about the influence of the Sun.
The moon attracts the Earth, just as the other way around. The waters at the side facing the Moon will be lifted, creating high tide. But at the other side of the Earth, there will also be high tides, because there the Moon is pulling less, so giving the water more 'room' to lift (away from the moon).
This explains, why there are always two tides a day (as the earth turns).
Now enter the Sun. It has a similar effect, although much smaller. If the Sun is in line with the moon, it amplifies the effect of the moon -- and it doesn't make a difference whether the order is Sun-Moon-Earth or S-E-M, because the tide also works at the other side of Earth (as seen from moon or sun). We call this a spring tide (higher than normal).
If the sun is at right angles to the moon -- as seen from earth -- it counteracts the working of the moon, as it 'tries' to make a high tide at places where the moon is creating a low tide, and vice versa. So the work of the moon is partly undone and tides are lower than normal. We call that neap tide .
In both cases, the low tides will also be lower after spring tide and less low after neap tide.