These illustrate a phenomena known as spring tides. Around the new moon and full moon when the Sun, Moon and Earth form a line (a condition known as syzygy) the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum: this is called the spring tide. In English, that means a much higher high tide and much lower low tide. The opposite of this, that is when the tide's range is at its minimum, is a neap tide.
An interesting animation to further explain this is available here.
