To take a mouthful; to eat a small bite.
新しいケーキを見つけたので、一口食べるだけでもいい？
I found a new cake—may I just take one bite?
赤ちゃんが食べ物に慣れるために、まず一口食べる練習をする。
To get the baby used to food, they first practice by taking one mouthful.
おいしそうな料理を見て思わず一口食べると、味が濃かった。
I couldn't help taking a bite of the delicious-looking dish, and it was strong in flavor.
彼はダイエット中だが、友達のケーキを一口食べるだけにした。
He is on a diet but limited himself to just one bite of his friend's cake.