This verse describes a scene where Jesus is engaging in a debate with some Sadducees about the resurrection (the preceding verses). A scribe, a Jewish legal expert, observes this debate and is impressed with the wisdom and skill Jesus displays in answering their challenging questions.

Here's a breakdown of the verse's meaning:

"One of the scribes came, and heard them questioning together." This establishes the context. A scribe arrives and listens to the ongoing discussion between Jesus and the Sadducees.
"Knowing that he had answered them well..." The scribe recognizes that Jesus has handled the Sadducees' arguments effectively. He's impressed by the clarity and depth of Jesus' reasoning. The phrase "answered them well" implies not just giving correct answers, but also doing so convincingly and thoughtfully.
"...asked him, “Which commandment is the greatest of all?”" Because the scribe is impressed by Jesus' understanding of the Law, he poses a direct and significant question. He wants to know which commandment Jesus considers to be the most important out of all the commandments in the Torah (the first five books of the Hebrew Bible). This question was likely debated among the scribes themselves, and it reveals that the scribe genuinely respects Jesus' opinion and wants to learn from him. He's seeking a definitive answer from someone he perceives as a knowledgeable authority.

In essence, the verse sets the stage for a profound theological discussion. The scribe's question is not a challenge or a trick; it seems to be born out of genuine curiosity and respect. He wants to know Jesus' perspective on the relative importance of the numerous commandments in Jewish law. The following verses then record Jesus' answer, which is crucial for understanding his teachings about love for God and neighbor.