well this depends on many factors: your material's quality, sequencing, ...

Can you clarify your question? Are you asking what the scores represent, or what they are used for, or something else?

A quality score is produced for each base during the sequencing process and represents the probability that the base was called incorrectly. See here: http://www.illumina.com/documents/products/technotes/technote_understanding_quality_scores.pdf

Looking at the distribution of quality scores with a program like FastQC can help give you an idea of the quality of your RNAseq reads and if/how quality filtering should be done for your dataset.

I hope that is helpful.