Spontaneous (exothermic) chemical reactions often require a push from the addition of externally supplied energy. This energy is often called activation energy. What does activation energy actually do? What is the energy hill that the activation energy surmounts?
It essentially is the energy required to start breaking up the molecules involved. This happens through collisions. So, the activation energy is a measure of the amount of heat (via temperature, which is a measure of translational energy of molecules) required for the collisions to start breaking things up when they hit.
For instance, CH4 and O2 at 200K won't really react. This is because the translational energy is too low; when an O2 hits a CH4, energy is exchanged but not enough to break the bond in CH4 or O2.
The form of energy transfer is also important. It can enter different modes: translational-translational, translational-rotational, translational-vibrational, etc.. It's the T-V collisions that matter. Once enough energy is transferred from translational energy to vibrational energy, the bonds snap and reactions occur.