The thirtieth letter of the Russian Cyrillic alphabet. It is named мя́гкий знак(mjáxkij znak, “soft sign”) and it has no sound of its own, but instead modifies the preceding consonant, causing it to be pronounced soft (i.e., palatalized). In the alphabet, it is preceded by Ы(Y) and followed by Э(E).
Less commonly, it just has a traditional orthographic usage with no phonetic meaning (like Russian туш(tuš, “flourish after a toast”) and тушь(tušʹ, “India ink”), both pronounced [tuʂ], but different in grammatical gender and declension), feminine nouns ending with "ь". This rule only applies to nouns ending in ч(č), ш(š), ж(ž) and щ(šč). Verbs in the 2nd person singular end in -ешь, -ёшь, -ишь, the final ш(š) is pronounced as [ʂ].
Also, it has a function of "separation sign" ("разделительный мягкий знак"): in Russian, vowels after the soft sign are pronounced separately from the previous consonant and are iotated. Compare Russian льют(lʹjut, “(they) pour/cast”)[lʲjut] and лют(ljut, “'(he is) fierce”)[lʲut].