Being the definitive statement of doctrine adopted by the Church of England in the years between the Elizabethan settlement of 1559-1563 and the formal adoption of the Thirty Nine articles in 1571.
When Elizabeth I succeeded to the throne in 1558, she introduced a new Book of Common Prayer, passed various chunks of legislation such as the the Act of Supremacy 1559 (which made the monarch once again Supreme Governor of the Church of England and generally restored Protestantism as the official religion.
Matters of doctrine needed to be attended to; the Edwardian articles were dusted off; a few replaced, and an amended set of
Forty Two articles were presented to Convocation for their approval. Only Thirty Nine were passed; Elizabeth struck out Article XXIX, Of the Wicked, which eat not the Body of Christ in the use of the Lord's Supper, 1
in order to avoid offending the Roman Catholic party. Therefore the newly re-constituted Church of England had to make do with a mere Thirty Eight articles for the time being. 2
However in 1570 Pope Pius V excommunicated Elizabeth thereby removing any chance of compromise or reconciliation, and also removing any reason for Elizabeth to continue to placate the English Catholics.
Hence in 1571, Article XXIX was restored, a few other minor changes were made and the the Thirty Nine Articles were then passed by Convocation, and received the sanction of Parliament in 1571.
Hence the Thirty Eight articles are really just the Thirty Nine articles minus one.
(And just a small footnote to English history.)
1 Which of course, insisted on Communion in Both Kinds, anathema to all God-fearing Catholics.
2 Like many of Elizabeth's policy decisions, it was probably just a ploy to buy time