The transmission or gearbox in a Honda CB125E is a standard type found in many motorcycles, known as a constant mesh gearbox with the gear configuration being one down, four up. There’s a diagram of this near the gearshift lever:
The “one down, four up” refers to the gears in relation to the neutral position, which is just above first gear. From first, you can click the lever up part-way to select neutral or push it firmly to select second, then keep on pushing upwards to select third, fourth and fifth.
Oh, and “N” in the above image refers to neutral gear!