Tverdo stands twentieth, writes t, and carries the numeral 300 from Greek tau. Its name is the Slavonic word for firm, hard or solid.
The shape is the uncial tau and is unchanged. Its numeral value, 300, makes it one of the letters used in writing dates, and in the Byzantine reckoning from the creation of the world that Slavonic books use, the hundreds letters appear in every year number a reader will meet.
The name has an afterlife the letter itself does not. When the reduced vowels stopped being pronounced and their letters were left doing purely orthographic work, the Russian tradition renamed them by function: the back jer became the hard sign and the front jer the soft sign. The adjective in the first of those names is the same word that names this letter, and the terminology of hard and soft consonants, which is fundamental to the description of every Slavic language, runs through the same vocabulary.
Tverdo itself is also one of the two candidates for the second element of the letter shta, on the reading that treats shta as a ligature built inside Cyrillic rather than a sign borrowed whole from Glagolitic. That question is genuinely open, and it is recorded on the shta entry rather than settled here.