Sentence:  Since every locale is of the form \mbox{Sub}_\mathcal{E} (1) [subobjects of the terminal object in \mathcal{E}] for some topos \mathcal{E}, <WSD>locale</WSD> theory can be regarded as the fragment of topos theory concerning subobjects of 1. A subobject of 1 is a map 1 \rightarrow \Omega, which can reasonably called a truth value. In that sense, <WSD>locale</WSD> theory is the study of truth values. 
Meaning of the word marked with <WSD> tags:  A partially ordered set with the following additional axiomatic properties: any finite subset of it has a meet, any arbitrary subset of it has a join, and "distributivity", which states that a binary meet distributes with respect to an arbitrary join. (Note: locales are just like (frame)s except that the category of locales is opposite to the category of frames.)