所 (suǒ) is a particle that turns a verb phrase into a noun phrase, like 'what is V-ed' or 'the thing that is V-ed' (e.g., 所需要 'what is needed'). It also appears in fixed patterns like 有所 (somewhat) and 为...所... (passive voice, formal/written). This is a grammatical word with no direct English equivalent — do not confuse it with the adverb 其实 (actually, in fact), which is a different part of speech and meaning.