The schwa is the unstressed vowel that occurs systematically in many German and Dutch words. In normal spelling, it is usually written as e; in the phonetic alphabet, an e upside down. It sounds like a short unstressed uh. Historically it usually developed from a 'real' vowel, often an a.

Examples of systematic schwa in Dutch:

many words, especially nouns, end in schwa
zijde (silk), mode (fashion), hybride (hybrid)
most plurals of nouns are formed with -en
huis (house), huizen (houses) - the English -es has a schwa too, but it sounds like ih rather than uh. The -e in house is silent; a schwa isn't.
the plural forms of the verb have -en
ik lach (I laugh), we lachen (we laugh)
adjectives have a form with -e:
een klein huis (a small house), het kleine huis (the small house)
the perfect participle is formed with ge-
ik heb gelachen (I have laughed) - in English the e in -ed may have been a schwa, but it is totally silent now