
prall
plump / taut
Filled to the maximum and therefore firm and tightly stretched. Often used for fruit, balls, tires, or body parts.
Der Fahrradreifen ist prall mit Luft gefüllt.
The bicycle tire is filled full of air and taut.
plump / blazing
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plump / taut
Filled to the maximum and therefore firm and tightly stretched. Often used for fruit, balls, tires, or body parts.
Der Fahrradreifen ist prall mit Luft gefüllt.
The bicycle tire is filled full of air and taut.

blazing
In the phrase 'in der prallen Sonne' (in the blazing sun), it means being exposed to direct, intense, and unprotected sunlight.
Lass die Schokolade nicht in der prallen Sonne liegen, sonst schmilzt sie.
Don't leave the chocolate lying in the blazing sun, or it will melt.

full / vibrant
Used figuratively to describe intensity and fullness, especially in the phrase 'das pralle Leben' (life in all its fullness).
In der Großstadt pulsiert das pralle Leben.
In the big city, life in all its fullness pulses.
'Prall' (plump/taut) is often combined with things that have a round, full shape. Typical examples are plump fruit (cherries, tomatoes), a taut ball, or a wallet stuffed full.
Die Kirschen am Baum sind dieses Jahr besonders rot und prall.
The cherries on the tree are especially red and plump this year.
The combination 'in der prallen Sonne' (in the blazing sun) is a fixed expression. Here, 'prall' does not mean 'filled', but rather 'direct', 'intense', and 'without shade'.
Man sollte mittags nicht in der prallen Sonne Sport machen.
You shouldn't exercise at midday in the blazing sun.
The expression 'das pralle Leben' (life in all its fullness) is figurative and describes a life full of intensity, energy, and events. It is often used to characterize dynamic life in a city or in a lively place.
Wer das pralle Leben sucht, zieht nach Berlin.
Anyone looking for life in all its fullness moves to Berlin.
direct
Direct sunlight reaches something without a protective barrier. The phrase pralle Sonne also describes unprotected exposure to sunlight, but additionally emphasizes its strong intensity. Direkt therefore refers to the way the sunlight reaches something, while prall is especially appropriate when the exposure to the sun is noticeably intense.

Die Kräuter stehen mittags draußen in der prallen Sonne.
At noon, the herbs are outside in the blazing sun.
Die Kräuter stehen am Fenster in direktem Sonnenlicht.
The herbs are standing by the window in direct sunlight.