According to Einstein's formula E = mc2 mass is a type of energy. This means that energy can be transformed to mass and vice versa. In particle accelerators this is used to transform kinetic energy to mass in a particle collision. In this way new particles can be created in collisions of particles with high speed relative to each other.
In the search of new heavy particles it is important to be able to accelerate the particles in the accelerator to high energies. The higher the energy of the original particles, the heavier particles can be produced in the particle collisions. Therefore at LHC it is possible to reach much higher energies than on LEP, because you use protons instead of electrons.