NRSV14 Now the disciples[b] had forgotten to bring any bread; and they had only one loaf with them in the boat. 15 And he cautioned them, saying, “Watch out—beware of the yeast of the Pharisees and the yeast of Herod.”[c] 16 They said to one another, “It is because we have no bread.” 17 And becoming aware of it, Jesus said to them, “Why are you talking about having no bread? Do you still not perceive or understand? Are your hearts hardened? 18 Do you have eyes, and fail to see? Do you have ears, and fail to hear? And do you not remember? 19 When I broke the five loaves for the five thousand, how many baskets full of broken pieces did you collect?” They said to him, “Twelve.” 20 “And the seven for the four thousand, how many baskets full of broken pieces did you collect?” And they said to him, “Seven.” 21 Then he said to them, “Do you not yet understand?”
At first I thought Jesus means his caution from v. 15 (which otherwise remains unexplained), but the word still seems to refer to something they should have understood a while ago. While searching online, I found an interpretation that he meant the fact that he can conjure food - but what is here to misunderstand? The disciples saw Jesus do that, so it's obvious he was capable of that. And what does the number of baskets with bread have to do with that?
I also read about the significance of numbers 12 and 7 - e.g. numbers of tribes of Israel and gentile nations of Canaan respectively, but I still I have no idea what they are supposed to explain.