what does " Reliability" mean ? i see it in signals tab in mql5 web site

 
when i want to chose a signal provider , i found some with good reliability and some they not , can any body explain how it calculated ? 
 
MetaTrader 5 Trading Signals Redefined
MetaTrader 5 Trading Signals Redefined
  • 2017.10.17
  • www.mql5.com
A new version of the Signals service for the MetaTrader platforms has been launched on the MQL5.com website...