The "monkey tablet", a Neo-Babylonian cuneiform tablet identified by Farouk Al-Rawi, is important in several ways, as Al-Rawi and Andrew R. George explain in their article
"Back to the Cedar Forest: The beginning and end of Tablet V of the Standard Babylonian Epic of Gilgameš" (Journal of Cuneiform Studies, 66 (2014).
First, it refutes the hypothesis that MS (manuscript) H from Nineveh and the Late Babylonian MS dd from Uruk (both manuscripts of Tablet V) represented two different textual traditions.
The tablet identified by Al-Rawi is in Neo-Babylonian and has the same incipit as MS H. (Note: The monkey tablet being Neo-Babylonian makes it a later manuscript than both MS H and MS dd.)
Second, Assyriologists have long debated the relative order of MS H and MS AA, both from Nineveh. (Nineveh is where Assurbanipal had his libraries, which were destroyed in 612 B.C.)
George Smith, who first translated both fragments in the 1870s assumed the order MS H first, MS AA second. However, many Assyriologist in the 20th century claimed that MS AA belonged to Tablet IV instead of Tablet V.
However, the "monkey tablet" "proves incontestably that the text of MS AA continues that of MS H after a short gap, and that both are witnesses of cols. i and ii of Tablet V as it was known at Nineveh."
Third, and probably most importantly for lay readers, the "monkey tablet" contains lines that had been missing in all known manuscripts of Tablet V.
In his book Das Gilgamesch-Epos (C. H. Beck, 2008, page 19) Walther Sallaberger presents a table describing the level of completeness of the tablets that make up the Gilgamesh Epic.
In 2007 or 2008, the available manuscripts of Tablet V provided 21% of that tablet's complete lines; 50% of its lines were missing.
(As is often the case with clay tablets, lines may be incompletely preserved due to physical damage. Sometimes, part of a line that is missing on one tablet can be found on another tablet; in other cases, line continuations are inferred from lines that are repeated elsewhere in a manuscript. This still leaves many lines incomplete.)
For comparison, we have only 10% complete lines from Tablet IV (the worst case) and 91% complete lines from Tablet XI (the best case so far).
In Andrew R. George's translation of Tablet V (The Epic of Gilgamesh: The Babylonian Epic Poem and Other Texts in Akkadian and Sumerian, Penguin, 1999), the text breaks off after line 10 and resumes at what George assumed to be line 53.
Based on the new tablet, 20 lines could be added to fill part of that gap.
Gilgamesh and Enkidu have arrived at the Cedar Forest, which is described as a jungle with cicadas, pigeons etcetera.
There are even monkeys, which gave the tablet its name on various blogs and online articles; however, Al-Rawi and George don't use the term "monkey tablet" in their article.
In Benjamin R. Foster's translation (The Epic of Gilgamesh. Second edition, Norton, 2019), the lines about the monkeys go as follows:
Mother monkeys kept up their calls, baby monkeys chirruped.
Like a band of musicians and drummers,
They resounded all day long in the presence of Humbaba.
Note that I used Benjamin R. Foster's translation instead of Stanley Lombardo's because Foster is an Assyriologist while Lombardo is a Classicist.
For a full translation of the fragment, see Al-Rawi and George 's article.