When we talk, sounds usually split into two teams: vowels, where the mouth stays open (like the /ɑː/ in father), and consonants, where we block or squeeze the air (like /t/). A semi-vowel sits awkwardly between those teams. It sounds rather like a vowel — the air is not stopped — but in a word it behaves like a consonant, clinging to the edge of a syllable instead of sitting in the middle as the vowel does. English mainly has two: /w/ as in we and west, and /j/ (the sound, not the letter y) as in yes and you. Some books also treat /r/ this way. Teachers sometimes call them glides, because the tongue glides quickly into the next vowel. Though actually the name is a bit of a nickname: it is not half a vowel. It is a vowel-like sound doing a consonant's job. You will meet the word in pronunciation class, not in the street.