Literary adverb used before a number to give a rough estimate. Often found in formal or written contexts; in everyday speech 約 (やく) or 大体 (だいたい) are more common.
Approximately three thousand spectators gathered.
The castle has a history of some five hundred years.
The kanji 無 (without) and 慮 (thought/consideration) combine to suggest 'without careful calculation', hence 'roughly'. The exact historical derivation is uncertain, but the word has been used as a literary adverb for approximation.