Your blood oxygen level is a measurement how much oxygen is reaching your muscles. It is important because low levels mean that you will become easily fatigued during exercise. The more exercise you do, the better your blood oxygen levels will become.
A heart rate monitor can help show your fitness levels, enabling you to calculate a suitable intensity of exercise.
GPS enables global positioning, useful in map, geo-tagging or navigation apps.
An accelerometer is a sensor used to measure the linear acceleration of a device. A common application is detecting when a device changes between vertical and horizontal positions.
With a temperature sensor you can monitor changes in temperature to measure your exertion levels and avoid hyperthermia.
A compass is useful for gaming, maps, and navigation software.
This measures barometric air pressure. It can predict weather changes, for example a sudden drop in air pressure could mean a storm is coming. When calibrated correctly it can be used to determine altitude, which helps GPS devices to lock on quicker and with greater accuracy.
A gyroscope is a sensor that tracks the orientation of a device, more specifically by measuring the angular rotational velocity. Initially, they were built using a spinning rotor to detect changes in orientation, like twisting or rotation.
A cadence sensor measures the number of pedal revolutions per minute when you are cycling. It enables you to monitor how fast you are pedaling.