Spelling City describes the diphthong sound and gives great examples, as well as a game that can help English language learners.
https://www.spellingcity.com/diphthongs.html
“A diphthong is a sound formed by combining two vowels in a single syllable. The sound begins as one vowel sound and moves towards another. The two most common diphthongs in the English language are the letter combination “oy”/“oi”, as in “boy” or “coin”, and “ow”/ “ou”, as in “cloud” or “cow”. Diphthongs are usually introduced as spelling patterns in the primary grades.”