悉 is a bound form (b.f.) — it cannot stand alone as an independent word. It appears as a morpheme in compounds with two main meanings: 'all, complete' (e.g., 悉数 'all, every') and 'know, understand' (e.g., 悉知 'to know fully', 悉心 'with all one's heart'). You'll encounter it in formal or literary contexts.