Complex sentence: The initial "E-" is the Basque prosthetic vowel, added because no Basque word may begin with an "R -," and the final "-a" is the absolutive case suffix, used when citing a name.
Split into simpler sentences: The initial "e-" is the Basque prosthetic vowel, which was added at a time that no Basque word was allowed to begin with an "r -". The final "-a" is the absolutive case suffix, which is used when citing a name.