Cadence is the number of steps that you take per minute. Most running watches will automatically measure your cadence, however if you don't have one you can easily measure your cadence by counting how many times your left foot hits the floor in one minute and multiplying that number by two.
Generally, you should target a running cadence of 180 steps-per-minute. However, you cadence will fluctuate depending on the speed that you're running at -it will be slightly faster for your intervals sessions, and a little slower on your easy runs.
Cadence is a very useful metric to look at when thinking about running technique. Low cadence has a big impact on your form, with slow cadence often suggesting more of a heel strike and resulting in more force being put through each step. This can lead to a higher risk of injury.
Increasing your cadence will lead to less impactful steps and lower vertical oscillation, which will in turn help protect your body (particularly shins and knees!) and save energy, allowing you to run faster and further whilst avoiding injury. When looking to increase your cadence, focus on your body position, ensuring your hips are forwards and shoulders are back.
Many smart watches also have a metronome feature which you can play and try to run in time with the beat. If you have a Garmin you can access this feature by heading to Running > Options > Run Settings > Metronome > Turn on. If you do not have a smart watch you can play a metronome on YouTube such as this video or Spotify such as this song.
If your cadence is well below 180, look to improve gradually over time, aiming for 165/170 at first before progressing to 180 from there.