BMW 1 series is the name of a small car by BMW. The 1 is BMW's entry level model.
BMW 1 cars, alike many other BMW models, are named with three numbers, such as 118. The first number stands for the model (here: 1), while the second and third letter stands for the engine size (here: 1 and 8, meaning 1.8 liters). An ending "d" (such as in 120d) means a diesel engined car.