in basic science ( a book of bharti bhavan ) 8th standard it is writeen that electrons are not very tightly bound to atoms . also it is written that when a voltage is applied atoms move in a orderly fasion ................. what does this mean ?

Dear Student!
The structure of the atom is such that the positively charged protons and the neutral neutrons are at the central nucleus while the negatively charged electrons orbit around it. The positive nucleus holds these negative electrons.

But, in some elements which have high atomic number, there are more number of electrons and the orbit in which they revolve is far from the nucleus. They experience lesser force of attraction and they are so less bound to the atom that they can be called free.

They move randomly from here to there not in a unique fashion.

When voltage is applied, these electrons which are free (unbound to the atoms) start to drift in an ordered line and constitute current.