The energy supplied by the Sun by way of ultraviolet radiation gives the gases enough "activation energy" to react.The molecules of Hydrogen (H2) and Chlorine (Cl2) exist as relatively stable gases, and in a sealed container mixed together would not react.  However, adding sunlight, which contains ultraviolet radiation (uv), adds enough energy to break the Chlorine molecule apart, which results in two "free radical" Chlorine atoms  (meaning that there's now an electron in the far orbit of the atom that wants to react.)  Since atomic Chlorine is very reactive, it can break apart a neighboring Hydrogen molecule, and form Hydrochloric Acid:

Cl2 + uv light --> 2 Cl

2 Cl + H2 --> 2HCL