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"Ъ" is a letter in the Cyrillic alphabet used in Russian and some other Slavic languages that represents a hard sign, indicating that the preceding consonant should be pronounced hard rather than soft. It doesn't represent a vowel sound itself but serves as a grammatical marker, particularly appearing at the end of words or before certain vowels to modify how neighboring letters are pronounced.
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Article
Hard sign, from Alexandre Benois' 1904 alphabet book. It shows dub′ (oak), vjezd′ (entry) and syr′ (cheese).
The letter Ъ ъ (italics Ъ, ъ) of the Cyrillic script is known as er golyam (ер голям – "big er") in the Bulgarian alphabet, as the hard sign (Russian: твёрдый знак, romanized: tvjordyj znak, pronounced [ˈtvʲɵrdɨj ˈznak], Rusyn: твердый знак, romanized: tverdyj znak) in the modern Russian and Rusyn alphabets (although in Rusyn, ъ could also be known as ір), as the debelo jer (дебело їер, "fat er") in pre-reform Serbian orthography, and as ayirish belgisi in the Uzbek Cyrillic alphabet. The letter is called back yer or back jer and yor or jor in the pre-reform Russian orthography, in Old Russian, and in Old Church Slavonic.