Fetch length, along with the wind speed (wind strength), determines the size (sea state) of waves produced. The wind direction is considered constant. The longer the fetch and the faster the wind speed, the more wind energy is imparted to the water surface and the larger the resulting sea state will be.
The winds which travel from the East Coast of the United States and hit the west coast of Ireland have an extremely large fetch and would in turn produce very large waves if the wind speed was high and wind direction remained constant over the duration of the fetch.
If you blow across a cup of water you will notice small ripples form on the surface and hit the far edge. The fetch distance is the diameter of the cup and wind speed is how strong you are blowing. If you blow at the same strength over a much larger bowl of water the ripples will become larger when they hit the far side of the bowl as there is more distance and thus more time for the blowing to impart energy onto the water to create the larger ripples across it.