They're referring to free school lunches. That is, a lunch, at school, which is free. In the sense that it does not have to be paid for on the spot or by the child or family on account. i.e. part of the educational provision itself.

(carefully phrased to avoid "no free lunch" nonsense)

I understand, I meant what solid evidence that it is working and what is meant by that?

It improves educational, health etc outcomes significantly and unlike a lot of proposed interventions in education, it has solid scientific evidence backing it up. Specifically it significantly improves child health (e.g. fewer hospital visits), improves maths and English test scores, lowers the incidence of suspensions of students, and there's no evidence that free school lunch policies increase BMI in those with access to them. For the cost, it is a very effective intervention in improving educational outcomes.

https://www.jandonline.org/article/S2212-2672(23)00301-5/ful...

https://www.brookings.edu/articles/schoolwide-free-meal-prog...

https://onlinelibrary.wiley.com/doi/abs/10.1002/pam.22175