The phrase is a reference to the story of Esau and Jacob in the Old Testament (Genesis 25, 29-34)

Esau and Jacob were twins but being born first Esau was meant to inherit position and inheritance (i.e. become the leader of the family after the father died). One day Esau came back from a hunting trip having killed nothing and he was starving. He saw Jacob cooking a dish of lentils and he asked him for a plate of lentils. Jacob agreed to the sole condition Esau gave up his birthright in his favour in exchange. To which Esau agreed and thus lost his inheritance.
Esau gave up something that is very precious for a plate of lentils. In French we say Esau lost hos birthright for a plate of lentils, in English we say he sold his birthright for a “mess of pottage“.

Félix Éboué means that one must think twice over the consequences of our acts, it might be better to refuse the mess of pottage and to keep your birthright. In the context of his speech he is telling the young people he is talking to they must learn to distinguish what is worth fighting for and fight for those values.