伍 has several distinct uses. Its most common modern sense is the formal numeral for 'five', used on checks, contracts, and official documents to prevent forgery (like writing 伍佰元 instead of 五百元). Historically, it referred to a five-man squad in ancient Chinese military organization, and by extension can mean 'army' or 'ranks' in literary contexts. It is also a common surname (Wǔ). The numeral sense is the one you'll encounter most often in practical settings.