Why are Cyrillic letters ъ and ь called back jer and front jer?

Why are Cyrillic letters ъ and ь called back jer and front jer?

I know that German and English have rhotic words:

Gothic English

auso ear
hausjan hear
wasjan wear
mais more
dius deer
wesun were

Gothic German

is er
was war
weis wir
mis mir
þus dir

Could Proto-Slavic endings ъ and ь be a rhotacization of Proto-Balto-Slavic endings us and is ?