Translation:The bread is clean.
The notes in one of the alphabet sections have a chart that describes letters at the end of syllables and what they sound like. According to that chart ㅅ at the end of a syllable, is pronounced like ㅌ unless it's borrowed by the next syllable; which only happens when the next syllable starts with ㅇ. So in 개끗합니다 it's pronounced like 개끝합니다.
The ㅎ also makes a little airy sound, like an h in English, that duo doesn't put across very well.