A quality score (Q-score) is a prediction of the probability of an error in base calling, also called Phred score. The FASTQ format encodes phred scores for each base alongside the read sequences.
Illumina scores span from 1 to 40.
Most Illumina runs will generate >70-80%  of the bases with quality score of at least 30.A high quality score implies that a base call is more reliable and less likely to be incorrect.
For example, for base calls with a quality score of Q40, one base call in 10,000 is predicted to be incorrect.
For base calls with a quality score of Q30, one base call in 1,000 is predicted to be incorrect.The table on the left shows the relationship between the base call quality scores and their corresponding error probabilities.