I will explain your quote to the best of my knowledge:
The Messiah, son of Mary, was not but a messenger; [other] messengers have passed on before him.
This shows that 'Isa () was one of Allah's messengers and he had been given the same orders and privileges as any other messenger before or after him
And his mother was a supporter of truth. They both used to eat food.
This part shows that both' Isa and his mother Mary () were humans, as both act like humans Allah used the example of eating as we are told in the story of Ibrahim () that angels don't eat (see for example Verse (11:70) did Ibrahim recognize his visitors as Angels or not?). While Allah informs us that we are in need of food while he doesn't need to be fed (6:14).
Look how We make clear to them the signs; then look how they are deluded.
So Allah gave clear signs of them being humans while others claimed they are the son of God or the mother of God (or the mother of the son of God). So these people who made such wrong claims are deluded. This can easily be understood if you go a few verses back (5:72-75) and read it in the context of the claims made by the Christians while earlier verses address other misdeeds of Bani Israel.
The verse clearly is a statement against polytheism and a typical call for monotheism as Islam and the quran are teaching.
You may also read in tafsir ibn Kathir:
'Isa is Allah's Servant and His Mother is a Truthful Believer
Allah said,
(The Messiah, son of Maryam, was no more than a Messenger; many were the Messengers that passed away before him.)
'Isa is just like the previous Prophets, and he is one of the servants of Allah and one of His honorable Messengers. Allah said in another Ayah,
(He ﴿'Isa﴾ was not more than a servant. We granted Our favor to him, and We made him an example for the Children of Israel.) Allah said next,
(His mother was a Siddiqah)
for she believed in Allah with complete trust in Him. This is the highest rank she was given, which proves that she was not a Prophet. Allah said next,
(They both used to eat food)
needing nourishment and to relieve the call of nature. Therefore, they are just servants like other servants, not gods as ignorant Christian sects claim, may Allah's continued curses cover them until the Day of Resurrection. Allah said next,
(Look how We make the Ayat clear to them. )
making them unequivocal and plain,
(yet look how they are deluded away (from the truth).)
look at the opinions, misguided ideas, and claims they cling to, even after Our clarification and plain, unequivocal explanation. (Source: qtafsir)
