yer vs hard sign

yer and hard sign both are nouns.

Word	Noun	Adjective	Verb	Adverb
yer	Yes	No	No	No
hard sign	Yes	No	No	No

As nouns, hard sign is a hyponym of yer; that is, hard sign is a word with a more specific, narrower meaning than yer:

yer: Either of two letters in Cyrillic alphabets, which originally represented phonemically the ultra-short vowels in Slavic languages.
hard sign: The Cyrillic letter Ъ/ъ, which in modern languages serves to denote a hard (non-palatalized) consonant.

Other hyponyms of yer include back yer, front yer, soft sign, strong yer, weak yer.

yer (noun)	hard sign (noun)
Either of two letters in Cyrillic alphabets, which originally represented phonemically the ultra-short vowels in Slavic languages.	The Cyrillic letter Ъ/ъ, which in modern languages serves to denote a hard (non-palatalized) consonant.