The gravitational attraction to the moon causes the Earth's oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth itself is pulled towards the moon (and away from the water on the far side). Since the Earth rotates, this happens twice a day. These are tides.
As for the importance of monitoring tides, one reason is that commercial and recreational fishermen use their knowledge of the tides and tidal currents to help them improve their catches. Monitoring the tides is a very important part of their livelihood.
