> In general, glass front panels are near optimal because they conduct electric fields almost twice as easily as plastic panels.

This means is Glass (\$\epsilon _R =4.7\$) has **twice** the relative permittivity of plastic ( \$\epsilon _R =2.3\$ ) in the materials preferred here. This applies to both mutual and direct capacitance.

*What is free space permittivity ?* 
  - about 9 picofarads per meter ( ~ 9 pF /m means means 9 pF  would be the capacitance of two hands with a gap of \$1/9^{th}\$ of the hand width.    
*Why it is relative?* all insulators are dielectric but glass is still twice as much as plastic and more than 4 times air or a vacuum.

> ultra-thin glass panels as retransmission effects can occur   

This means a partial discharge (PD) can occur, which may damage cells from the E field breakdown as breakdown threshold for example  >10kV/mm or 10V/um is approached with ultrathin glass < 1mm