In Southern California we have two high tides and two low tides each day. Generally one of the high tides higher than the other (the "high" high tide) and one of the low tides is lower. They are caused by the gravity pull between the earth, moon, and sun. The time of the tides change by about 50 minutes each day as the moon migrates through its monthly cycle.

If you live somewhere else, search for closest coast to you at this website. The height of the tides are compared to an average local low, low tide.

So, a low tide of -0.1 ft is below average. A high tide of 5 ft is pretty high in this area. Some places (like Alaska) may have high tides of more than 20 feet!!
This site about tides has a pretty good description and pictures of how tides work. When you take physics some day, you will learn an even more detailed explanation.

Due to gravity, the sun and the moon are always pulling on the Earth and they cause the Earth to bulge at the equator. During a Full Moon, for example, the bulges will point towards the Sun and the Moon (which are on the opposite sides of the Earth at Full Moon). As the earth rotates over one day, each part of the Earth passes through these two bulges, which are the high tides. The low tides, then, are the times of day where a particular part of interest of the earth is not where these bulges are.