What does "remove it and then use it for the generation of electricity" mean in the sentence:
The energy released during the fusion of hydrogen is so enormous that it is very difficult remove it and then use it for the generation of electricity.
In the fusion reaction of hydrogen, two Hydrogen nuclei collide to form a Helium nucleus. As a secondary product of this nuclear reaction, energy is released because of the mass defect (mass of He nucleus < 2*mass of H nuclei). This energy is released in many forms mainly as kinetic energy of the resulted Helium nucleus (in other words heat) but there are also other form that the released energy can take (like electromagnetic energy).
As said above to increase the kinetic energy of constituent particles of a gas means to increase its temperature. If one wants to remove this energy that has just been released, one needs to cool down the gas and take out heat from it. One method to do this is by circulating a coolant agent through the medium where the nuclear reaction takes place (for example to circulate water in the space between Uranium bars that undergo nuclear fission). The water transforms into steam, and this steam is then used to drive a turbine that generates electricity.
Unfortunately, heat generated in the fusion reaction can not be removed so easy because of two reasons. First reason is that a large part of it is needed to maintain the fusion itself (incoming hydrogen nuclei need to have enough initial kinetic energy to collide with each other), and the second reason is that the temperature of the reaction is so big that it is almost impossible to circulate a coolant through the plasma gas where the fusion reactions take place.