Prokaryotic ribosomes begin translation of the mRNA transcript while DNA is still being transcribed. Thus translation and transcription are parallel processes. Bacterial mRNA are usually polycistronic and contain multiple ribosome binding sites. Translation initiation is the most highly regulated step of protein synthesis in prokaryotes.
The rate of translation depends on two factors: 
* the rate at which a ribosome is recruited to the RBS
* the rate at which a recruited ribosome is able to initiate translation (i.e. the translation initiation efficiency) 
The RBS sequence affects both of these factors.