The Apocrypha is a collection of fourteen books that had traditionally been included by Christian churches as part of their scriptural canon. However, by the 1500s there began to be discussion about whether these books were really scriptural. Over time, many Protestants, while reading them as edifying literature, rejected them as being canonical. At the same time, the Catholic and Eastern Orthodox retained them in their scriptures. In the King James Bible that Joseph Smith was using for his bible translation there was a separate section from the Old and New Testaments, called the Apocrypha, that included these fourteen books.
As part of his Bible translation, when Joseph got to this section, he asked the Lord if he should translate it. In response, Joseph received the following revelation: “There are many things contained therein that are true, and it is mostly translated correctly; There are many things contained therein that are not true, which are interpolations by the hands of men. Verily, I say unto you, that it is not needful that the Apocrypha should be translated. Therefore, whoso readeth it, let him understand, for the Spirit manifesteth truth; And whoso is enlightened by the Spirit shall obtain benefit therefrom” (Doctrine and Covenants 91:1–5). So it looks like that although it contains some errors, and Joseph Smith was told not to translate it, the Apocrypha is “mostly translated correctly,” contains “many things…that are true” and if we’re enlightened by the Spirit we can “obtain benefit” from it.
Here are a couple of links that can help you if you’re interested in studying the Apocrypha:
One of my favorite books in the Apocrypha is 1 Maccabees. Here’s a link to chapter one of the NRSV version.
My colleague Jared Ludlow has written an accessible book that I highly recommend: Exploring the Apocrypha from a Latter-day Saint Perspective.