Water pressure is simply the force which pushes water through your pipes and taps.  The speed with which water flows through pipes and taps is called the flow rate.

Your household water pressure is determined by a number of factors, these are mostly beyond your control and are the responsibility of your local Water Authority or water supplier.  In the UK, water companies are legally obliged to provide specified minimum standards of pressure and water quality.  You can read more detail here. (Ofwat is the government regulator for water suppliers in England and Wales.  Legislation in other areas may vary)
 

Water flowing into your home, referred to as the incoming main, is regulated by a stop cock.  This can reduce the flow of water and even turn it off completely, so if your pressure is low you can check if your stop cock is full open.  It just works like a big water tap, turn the handle clockwise to close it and anti-clockwise to open it up.  Its a good idea to familiarise your self with this piece of kit, if you ever have a burst pipe or a leak this is where you can turn off the incoming water supply, so ensure you know where it is.

The stop cock is often found in the cupboard below the kitchen sink, but this is not always the case, so make sure you know where yours is, and check it works.  Often in older installations the stop cock can be very stiff and hard to turn.  Find it, check it and if you ever have a burst you will know where to turn off the water.
 

In a lot of houses have a tank fed hot water supply where water is fed from a header tank in your roof-space.  This is just a big tank which is constantly being topped-up from the incoming mains cold, as water is fed to the hot cylinder below and onwards to the hot taps when needed. 

 

 

 

 

Cold water is also fed directly from the header tank to the cold taps, except for the kitchen cold tap which is fed direct from the mains.  The difference in height between the header tank and a tap, referred to as the head of water, determines the water pressure, and as this distance is rarely more than a couple of meters, the water pressure will be low.

 

This type of tank fed system is also known as open vented and is typical of many installations in the UK, particularly in older properties.  It originates from the time when most homes were heated by an open fire and a back boiler heated the water.  While it works perfectly well with various other boiler systems, the water pressure will always be low.  This can cause difficulties with modern mixer taps and particularly with shower systems in the bathroom, but if this is your hot water system, don't worry, there are solutions to help increase the pressure.
 

1bar pressure is roughly equivalent to a 10M head of water, so where the head is only 2M the pressure will be only 0.2 bar. 

To give you a comparison, the mains pressure is usually around 3 bar.

The relationship between pressure and flow rate is a little complex, but think about trying to push water down a pipe.  If the pipe is wide it doesn't require much force to push water through, but if the pipe is narrow it takes a lot more effort.  So for a given pressure, the flow rate will increase with the bore of the pipe.  This concept will help you to understand why some taps work better than others on low pressure, and also why showers are very much affected by water pressure.