In this lesson, the focus is on number systems, specifically aimed at fourth-grade secondary school students. The teacher explains how to find the largest number that can be stored in a 4-bit register and introduces the concept of binary representation. The lesson covers converting binary to decimal using the place value system and emphasizes that the register consists of four bits, which can each hold a value of either one or zero. Students will learn how to determine the largest binary number possible within the constraints of a 4-bit register.