What does term slightly increased cortical echogenicity of both kidneys means in an ultrasound test?

"Slightly": increased echogenicity is a hedge. It means that the kidney tissue may be marginally brighter looking than normal on US. It could be normal. Truly increased echogenicity implies renal disease but is very nonspecific, there are a number of entities that could manifest that way. The finding must be interpreted in the context of your clinical picture to determine the significance, if any.