"She is eating beef."
Translation:Elle mange du bœuf.
The partitive article is used for unspecified amounts of uncountable nouns. In English, it can translate to "some", but it's often just omitted. Even though English speakers understand this implicitly, in French it is necessary to state that fact overtly, so it is required to use the partitive. Remember that du is a contraction of de + le and that partitives can elide.
See this lesson for a full explanation: https://www.duolingo.com/skill/fr/Food