It's the probability of a shot resulting in a goal, with 0 being a shot that could never go in and 1 being a shot that could never miss. A penalty is about 0.75 xg for example. A 1.2 xg shot would be a shot that has a 20% chance of scoring two goals, which would be impressive!
Expected goal assigns a value to each shot between 0.00 and 1.00 to reflect the probability that it will result in a goal. A shot that measures 0.01 xG suggests it could reasonably be expected to result in a goal one in 100 times. In other words, there is a 1 per cent chance it will go in so not a very good chance.
This value represents the probability of the chance resulting in a goal, on a scale of 0 to 1. For example, a shot with an xG value of 0.1 would be expected to result in a goal 10% of the time, while a shot with an xG value of 0.9 would be expected to result in a goal 90% of the time.