Flow is the amount of water (in m³ or litres) that flows from your tap during a given time (per second, per hour). The longer it takes to fill a container (a sink, a bucket, …), the lower the flow.
Pressure is a force exerted by water on a surface. To check if you have pressure, try stopping the water supply by plugging the tap with your finger. The easier it is, the less pressure there is. In drinking water distribution networks, pressure is usually measured in bar.