This is the 730th most frequent Hungarian word.
szigorú
Strict.
In this sentence, 'szigorú' is an adjective describing the teacher's personality, specifically how strict or rigid the teacher is.
Az új tanár nagyon szigorú a tanulókhoz.
The new teacher is very strict with the students.
In this usage, 'szigorú' characterizes the diet as being rigid and disciplined, enabling success.
A szigorú étrend segített elérni a célomat.
The strict diet helped me achieve my goal.
Here, 'szigorú' refers to the quality of the rules, describing them as strict, implying they allow little flexibility.