On an ecg, what does a "persistent s-wave ( v5-v6 )" mean?

Not much: on the ECG, the first downward deflection that follows the dominant upward deflection is called an "S" wave and is part of the depolarization complex (called a "QRS complex"). Classically, the S wave is tiny or absent in V5-6. When the S wave is deep, the term "clockwise rotation" is used. (your heart is rotated in your chest) It doesn't connote any pathology. Not to worry.