Full wave rectifier "Though it is unidirectional it does not have a steady value" the statement has been  mentioned in ncert .I just wanted to why the current is not steady in full wave rectifier though it is a dc current and how can filter like capacitor can provide pure dc voltage ? And what does  pure dc voltage actually mean?
 A: 
I just wanted to why the current is not steady in full wave rectifier
  though it is a dc current

A full wave rectifier without a filter will have a time varying, but unidirectional, voltage and load current. The magnitude of the dc current for full wave rectification is its average value. For a rectified sinusoidal current it will be about 0.637 of its peak value.

and how can filter like capacitor can provide pure dc voltage ?

A capacitor put in parallel with the output of the rectifier will charge up and initially acquire a voltage equal to the peak voltage output of the rectifier. With a current load the capacitor may discharge (lose some voltage) between cycles, the amount of discharge depending on the capacitance of the capacitor and the amount of load current. The larger the capacitance and/or lower the load current the less the amount of discharge and the closer the capacitor will maintain the output voltage to the peak value. 

And what does pure dc voltage actually mean?

Pure dc voltage means there is no time varying component of the voltage and current. In the case of the rectifier, that would require the capacitor to maintain the output voltage equal to the peak rectifier output, with no "ripple" voltage and current.
Hope this helps.
A: Basically, when the current passes through a full wave rectifier, the magnitude varies in the same way, but in every half cycle the direction is reversed.
Due to this it has a variable magnitude but constant direction. Thus, it is a direct current.
I think by 'pure' DC you mean the one which has a constant value. So basically the filter makes current magnitude change more even over time, and thus we get a more 'pure' DC with almost constant magnitude.
