explain equilibrium in terms of the collision theory
It depends on the reaction or change you want to describe.
In many reactions, if reactants collide with sufficient incoming energy, they form a new compound, with a relase of energy (it, the new particle is vibrating, rotating, or translating). The reverse reaction if somehow energy is released, through a collision, or other, the compound breaks up into reactants. Both of these reactions, forward, and reverse, depend on temperature, pressure...so eqilibrium is when both reactions have the same rate.