The schwa (sjwa in Dutch linguistics) is the neutral mid-central vowel — the sound of the unstressed e in words like de, een, -en plural endings, and in many unstressed syllables. It is written as e but sounds like a very short, neutral uh. In de fiets, the de is a schwa. In spoken Dutch, unstressed syllables collapse to schwas at high speed — this is one reason fast Dutch speech sounds so different from careful textbook pronunciation.
Where schwas appear: plural ending -en (huizen — houses, the -en is a schwa + n, often reduced to just n in fast speech), prefix be- (begrijpen — the be- is a schwa), de and een articles in unstressed positions, and many unstressed syllables in longer words. In very fast speech, entire syllables disappear: waarom (why) becomes waarom or even warom; eigenlijk (actually) becomes eigelijk or eegelijk.
Why schwas matter for listening: if you expect to hear full vowels in every syllable, fast Dutch speech will sound like random noise. Once you learn that unstressed e → schwa → sometimes nothing, connected speech starts making sense. For production: do not overpronounce unstressed syllables. De fiets with a full ee vowel in de sounds as stilted as carefully pronouncing every letter of the in English. Relax unstressed syllables into schwas and your Dutch will sound notably more natural.