adj. saf·er, saf·est
1. Secure from danger, harm, or evil.
2. Free from danger or injury; unhurt: safe and sound.
3. Free from risk; sure: a safe bet.
4. Affording protection: a safe place.
5. Baseball Having reached a base without being put out, as a batter or base runner.
1. A metal container usually having a lock, used for storing valuables.
2. A repository for protecting stored items, especially a cooled compartment for perishable foods: a cheese safe.
3. Slang A condom.
your instructor is probably referring to definition #3
photograph and ( art in general )
is about taking risks.
i am guessing your still lives are pretty nice
but you could probably do something out of the ordinary,
juxtaposition, or point of view or anything else ...
or as other people have suggested .. it is all about "comfort zone "
maybe your instructor wants you to photograph a situation where you have to interact
with the subjects .... architecture, portraits ...
good luck !