Same word, different gender

A few French nouns change their meaning with their gender, so un or une tells you which word is meant:

• un livre (a book) / une livre (a pound, half a kilo)
• un tour (a trip round, a turn) / une tour (a tower)
• un poste (a job, a position) / une poste (a post office)
• un mode (a way, a method) / une mode (a fashion)
• un manche (a handle) / une manche (a sleeve)
• un moule (a mould, a cake tin) / une moule (a mussel)
• un mémoire (a dissertation) / une mémoire (a memory)
• un voile (a veil) / une voile (a sail)

Read the whole sentence: the context decides the meaning, and the meaning decides the gender.