Allah (swt) talks about the Jews, the people of the Children of Israel, and about their violation of the covenants and promises they made in Medina. This is a reminder of their ancestors and their shameful past with the Prophet Moses (PBUH), and of their many requests and denial despite all that Allah (swt) revealed to Moses (PBUH).
(… فَبِمَا نَقْضِهِم مِّيثَاقَهُمْ ) (And [We cursed them] for their breaking of the covenant …): The Arabic linguistic context refers that they used to break their covenants times and times, not one time. This is one of the very precise expressions in the Holy Quran.
(… وَكُفْرِهِم بِآيَاتِ اللَّهِ …) (…and their disbelief in the signs of Allah): They disbelieved in the evidences that were given to them such as splitting the sea and the drown of Pharaoh, sending down the manna and quails,… Despite all these gifts from Allah (swt) they disbelieved in His existence and asked for proofs, moreover, they killed the Prophets as Yahya and Zechariah (PBUT):
كُلَّمَا جَاءَهُمْ رَسُولٌ بِمَا لَا تَهْوَىٰ أَنفُسُهُمْ فَرِيقًا كَذَّبُوا وَفَرِيقًا يَقْتُلُونَ
(Whenever there came to them a messenger with what their souls did not desire, a party [of messengers] they denied, and another party they killed.) (Al-Maida: 70)
(… وَقَوْلِهِمْ قُلُوبُنَا غُلْفٌ …) (…and their saying, “Our hearts are wrapped”…): They said that their hearts are covered with disbelief in it so faith cannot enter them, rather Allah sealed them because of their disbelief after He has showed them the right path but they chose the way of disobedience.