Capacitor is made of say two plates seperated by dielctic 
material. When volage is apllied the ions on the plate are 
zero so initially more current will flow like in short 
circuit.
As time passes the charges(+ve and -ve) accumalate on 
plates over a time and it opposes current flow. Now we say 
capacitor charged fully. Now the time taken to charge fully 
is called time constant of a capacitor. This time can be 
varied by connecting a resistor in series with capacitor. 
When high resisor is connected voltage drop across 
capacitor is reduced so it take long time to charge fully.
Vica-versa for low resitor. The relation is exponetial 
charging and discharging. Discharging process is also 
similar way.