French has three definite articles depending on gender and number: le for masculine singular nouns (le chat, the cat), la for feminine singular nouns (la maison, the house), and les for all plural nouns regardless of gender (les chats, les maisons). Unlike English "the", which never changes, the French article carries information about the noun itself — one reason French nouns are always learned together with their article.
Before a vowel sound or a mute h, both le and la contract to l' (l'ami, the friend; l'heure, the hour). This is purely about pronunciation — a run of two vowel sounds is avoided — so it applies even though the noun's underlying gender hasn't changed; you still say un ami and une amie once a different determiner is needed.