The Moon attracts the water in the seas, pulling up a bulge of water towards it.
The Earth itself is also attracted and so another bulge of water is ‘left behind’ on the opposite side of the Earth from the Moon. Places on the Earth beneath the two bulges have high tides at that moment while places at right angles to these have low tides at that moment.

The Earth rotates beneath these two bulges and as it does so every place on Earth has two high tides and two low tides a day.

There are not exactly 12 hours between the high tides at a particular place because the Moon is both pulling on the water and moving round the Earth. As it moves it 'pulls’ the water bulge round with it.