The earth completes a full rotation once per day, and hence the moon passes overhead pretty
much once daily.
So then why are there two high and low tides every day?
Simple: the moon's gravitational attraction is stronger the closer you are to it.
So the close side of the earth gets pulled away from the middle, which in turn gets pulled
away from the far side.
As the earth rotates, these so-called "tidal forces" cause water on the surface to slosh
around as "high tides" twice per day - when the moon is directly over your head and directly
below your feet.
So why is gravity weaker at a greater distance?
Well, imagine you're spraying a stream of water at a sheep [flamingo].
If the sheep's more distant, less of the water hits it than when it's close, so there's less
force pushing it away.
Gravity behaves the same way, except it's attractive.
And there are tidal forces from the sun as well: they're weaker than the lunar ones,
but when the sun and moon line up [on the same or opposite sides of the earth] the tides
pack an extra punch.
It's not just water that sloshes, though - tidal forces cause the earth's crust to bulge as
well - we just don't notice because we get lifted along with it.
And tidal forces from the earth cause the moon to bulge, too!
All this bulging and sloshing dissipates some of the earth's rotational energy, causing
it to gradually slow its rotation.
Eventually, the earth will spin slowly enough that the same side of the earth will always
face the moon - just as the same side of the moon already always faces the earth!
This means that in the distant future, a day and a lunar month will each have the same
length: about 50 of our current earth days.
What's more, if the moon were to get too close to us, tidal forces from the earth would be
stronger than the gravity holding the moon together, so rocks on the moon's surface would
literally start flying off into space, and the moon would disintegrate.
This would also happen if the earth were to get too close to the sun - and if you decided
to fly into a black hole, it's what would actually do you in: your spaceship would get
ripped apart from tip to tail by tidal forces.
Now that's bad tidings.