Deciles are another way we can consider location of data, where the data is separated into 10 groups. Just as quartiles correspond to specific percentiles, so do deciles. That is, the first decile is equivalent to the 10th percentile, the 5th decile is equivalent to the 2nd quartile and the 50th percentile.
If you are asked to find a specific decile, you are really looking for the corresponding percentile (multiply the decile by 10 to get the equivalent percentile). The 8th decile is equal to the 80th percentile.
You can also think of these measures of location like money: 2 quarters (2nd quartile) = 5 dimes (5th decile) = 50 pennies (50th percentile).