Retroflex sounds are made by curling the tongue back so that the underside of the tongue touches the area just behind the teeth (the alveolar ridge).
Sounds like [n], [d], and [l] are normally made with the tongue tip touching the alveolar ridge, but retroflex sounds use the underside of the tongue touching the same place.
Retroflex sounds have a ‘rhotic’ quality – like an ‘r’ sound.
These can be difficult for speakers of Australian English, who tend to ignore ‘r’ sounds before consonants. It’s important to make these sounds as retroflex, as you might be saying a completely different word if you don’t curl your tongue back.
Retroflexes are very common in Australian Indigenous languages, but they can be spelled in different ways. For example, Yolngu matha uses underscored letters: ḻuku, maṉḏa, waṯu, as does Pitjantjatajra: Kata Tjuṯa, Aṉangu, while Bininj Kunwok and Warlpiri use a digraph such as wurdurd (children – K), warna (snake – W).