Learn French: Elle voyage toute l'année.

Because tout has to agree with the noun it describes. Here, it describes année (year), which is a feminine singular noun. Because of this, the masculine tout changes to the feminine form toute.

In English we just say "all year", so why is there an l' in toute l'année?

In French, when you use tout (or its variations like toute, tous, toutes) to mean "all" or "the whole", you must include the definite article (le, la, l', or les) before the noun. You cannot just say toute année; it has to be toute l'année (literally "all the year").

Why does this sentence use année for "year" instead of an?

French has two words for year. An is mostly used for counting them as simple units (like Elle a trente ans - She is thirty years old). Année focuses on the duration or the actual span of time. Because she is traveling throughout the duration of the year, toute l'année is the correct phrase. (In fact, the phrase "tout l'an" isn't used in everyday French).

How do you pronounce toute in this sentence?