Strictly speaking it means very little, because most reactions have an activation energy, so most reactions require some amount of thermal energy to proceed. There are really two possible implications that are relevant: (a) It distinguishes the fact that the reaction as described in the current context is driven by thermal energy as opposed to, for example, light energy; (b) the reaction in its current context is driven by thermal energy and the rate is practically zero at ambient temperature - i.e., a supplemental heat source needs to be supplied in order to observe formation of products over practically relevant timescales.