Essentially, the spread in forex is the difference between what you buy or sell a currency pair at, vs when you actually start making some profit.
For example, if you were to place a BUY trade in EURUSD and the spread is 1 pip. The pair would have to move up more than 1 pip before you start to realise profit.
TIP - you can see the current spreads on all currency pairs on your brokers website. They are normally in the range of 1-3 pips depending on the currency pair.