Japanese uses a set of sounds that combine a consonant with a "y" glide, similar to the "ky" or "hy" sounds in English words like "cute" or "human". In kana, these palatalized sounds are written by combining a base consonant with a smaller version of one of the Y-row characters. A small や, ゆ, or よ is attached to the lower right of the preceding kana, forming a single syllable. This construction is called yōon, meaning "contracted sound".
Yōon exists for every consonant that can pair with "y". From the K-row comes きゃ (kya), きゅ (kyu), and きょ (kyo); the voiced version gives ぎゃ (gya), ぎゅ (gyu), and ぎょ (gyo). From the S-row comes しゃ (sha), しゅ (shu), and しょ (sho), with the voiced variant じゃ (ja), じゅ (ju), and じょ (jo). From the T-row comes ちゃ (cha), ちゅ (chu), and ちょ (cho). The N-row gives にゃ (nya), にゅ (nyu), にょ (nyo); the H-row gives ひゃ (hya), ひゅ (hyu), ひょ (hyo); the M-row gives みゃ (mya), みゅ (myu), みょ (myo); and the R-row gives りゃ (rya), りゅ (ryu), りょ (ryo). The H-row also has voiced forms びゃ (bya) and half-voiced forms ぴゃ (pya).
In katakana, yōon uses the same idea but with angular shapes: キャ, シャ, チャ, ニャ, ヒャ, and so on. The second character is always written smaller than the first, which is the visual cue that the two kana together represent one sound. A useful example is the place name とうきょう, which contains きょう, a two-kana yōon that counts as a single mora. Understanding yōon unlocks a large portion of the Japanese sound system that the basic 46 characters alone cannot represent.