"She is eating bread."
Translation:Elle mange du pain.
because "elle mange le pain/she is eating the bread" means something else.
the nuance is as follows:
she is eating the bread = this bread, the bread on the table, a defined bread => definite article THE/LE
she is eating bread = some bread, a piece of bread, an undefined quantity of bread => partitive DU
Not really the same as that, a/an is just tied to whether the noun begins with a vowel (and as a sidenote the "n" in "an" has actually migrated in some cases with "a nadder" and "a napron" becoming "an adder" and "an apron").
But some other French prepositions contract as well: de+les =des, à+le =au, and à+les =aux.