Au moins and moins are two different expressions. They both refer to quantity, but they do not have the same meaning.
Au moins
Au moins means: at least, not less than.
- Il y a au moins 10 personnes. (There are at least 10 people.)
- Attends au moins 5 minutes. (Wait at least 5 minutes.)
- J’ai au moins 200 euros. (I have at least 200 euros.)
Moins
Moins means: a smaller quantity, less.
- Je veux moins de sucre. (I want less sugar.)
- Il y a moins de monde aujourd’hui. (There are fewer people today.)
- Elle travaille moins que lui. (She works less than he does.)
Compare
- Il y a au moins 20 élèves. → minimum 20 (There are at least 20 students. → minimum: 20)
- Il y a moins de 20 élèves. → nombre inférieur à 20 (There are fewer than 20 students. → fewer than 20)
- Reste au moins une heure. → minimum (Stay at least one hour. → minimum)
- Reste moins d’une heure. → durée plus courte (Stay less than one hour. → shorter duration)
Simple difference
- Au moins → minimum
- Moins → smaller quantity