Spread is the difference between the bid (sell) and ask (buy) price.
To understand, let's recall the currency exchange points. There we always see the difference between the purchase price of a currency and its sale price. This difference is the earnings of the bank, which buys and sells this currency. When trading through a broker in the FOREX market, we will also see this difference, but not as much as in exchangers, but much smaller. For example, a quote of “EUR 1.2900 / 1.2903” means that the broker is currently ready to buy from us the euro against the US dollar at 1.2900, sell you the euro at 1.2903, and the difference between the purchase price and the sale price will be 3 points (0.0003). This is the spread.
To summarize the above mentioned, the spread is the difference between the buy price (Ask) and the sale price (Bid).