What does dilated fetal kidneys with normal amniotic fluid mean for a second trimester fetus?

No big deal. Detecting renal problems by fetal ultrasound is very accurate and efficient because it can prevent serious complications postnatally. Fetal renal pyelectasis or even hydronephrosis is nothing to worry about, provided the amniotic fluid is monitored and remains normal. The neonatology team should be notified at birth to institute appropriate follow-up for the newborn; monthly ultrasounds are needed.
Hormonal effect. Mild dilatation of kidneys is often the effect of maternal Progesterone on the baby. If the amniotic fluid is normal chances are that there's no significant disease affecting the fetus. Careful observation and follow up ultrasound is indicate in consultation with the OB team.