This is the 1st most frequent French word.
de
"De" in French means "of" or "from" in English.
'De' is used to indicate the contents (mysteries) of the adjective 'plein' (full).
Ce livre est plein de mystères.
This book is full of mysteries.
The usage of 'de' links the noun 'tasse' (cup) with the content it holds ('thé', tea), representing composition or containment.
Je voudrais une tasse de thé.
I would like a cup of tea.
Here, 'de' introduces the noun 'sommeil' following the phrase 'avoir besoin de', which traditionally pairs with 'de' to specify the requirement.