Whenever a reaction occurs, energy is traded (gained or lost) and the disorder of the system increases or decreases(entropy).
The change in gibbs energy is defined as del(G)=del(H)-Tdel(s). Now, if this quantity is positive, i.e. del(h)>Tdel(s), then that particular reaction is not spontaneous. You will have to provide energy to make it happen.
On the other hand, if del(g) is negative, then the reaction happens spontaneously. You dont need to do anything to make it happen.
This is why endothermic reactions are possible. Even if they absorb energy, (del(H)>0), the increase in entropy (or disorder: del(s)) is great enough so that del(h)<Tdel(s), which results in the change in gibbs free energy being negative. Hence, the reaction proceeds spontaneously. The evaporation of water is an example of this.
Note, however, even if the reaction is feasible, but the entropy of the system decreases, the total entropy (system+surrounding) will always increase for any feasible process, as stated by the second law of thermodynamics.