It works by you betting against a 'spread' which covers an outcome and consists of two prices, a 'buy' price and a 'sell' price. An example may be a predicted spread of 10 - 11 corners in a Premier League match.
A +1.5 spread is commonly seen in baseball betting, the standard runline for MLB. This spread means the underdog must win outright or lose by exactly one run to cover the spread. Alternatively, a -1.5 spread means that the favorite must win by at least two runs. Many baseball games are decided by fewer than two runs.