The forex market is the most liquid market in the world with an average daily trading volume of 5 trillion dollars. So, the spreads are very low in such a market. When you want to sell something, you need a buyer. When you want to buy something, then you need a seller. Hence, forex market is one where there are always enough number of buyers and sellers with low spreads.
When there are more buyers and sellers in the market, it is called as a liquid market. The major pairs such as the EUR/USD, GBP/USD, etc. have high liquidity. There are many buyers and sellers for these pairs to buy or sell.
For scalpers and intraday traders, liquidity is very important. So, look for times of high liquidity such as news events to gain the most from price movements to profit.