The area under the normal curve to the left of a given z-score represents the probability of all values below that z-score occurring. This area can be found using a z-table that represents the area under the normal curve to the left of z. Once you have located the z-score in the table, you can find the corresponding area. This will be the area under the normal curve, to the left of the z-score.
For a z-score of 0.9, you would need to look up 0.9 in a z-table to find the area under the normal curve to the left of that value.