Nouns ending in -et are masculine.
The suffix often forms a diminutive or small object.
Is sujet masculine or feminine?
lesujet
The rule behind it
99.2% of -et nouns are masculine
In context
Puis-je vous parler de ce sujet en privé ?
May I talk with you in private about the matter?
Ne vous appesantissez pas trop sur le sujet.
Don't dwell too much upon the subject.
J'ai concentré mon attention sur le sujet.
I concentrated my attention on the subject.