Almost exclusively used in the set phrase つぶらな瞳 (round, cute eyes). Describes eyes that are large, round, and endearing.
Her round, cute eyes were striking.
When the puppy looks at me with its round, cute eyes, I can't help but spoil it.
Kanji spelling is uncommon; the kana form つぶら is strongly preferred in modern usage.
The origin is uncertain. It may be related to 円 (つぶ) meaning 'round', but the exact derivation is unclear.