Using a heart rate monitor (HRM) is a great way to tune into what is going on inside your body as it relates to your running performance.
If you Google "heart rate training," it won't be long before you're so confused that you'll want to pack up monitor and ship it back. I'd avoid this at all costs. The truth is: Training by effort doesn't have to be so darn complicated. It's less about tricky formulas and numbers and more about what your body is telling you.
The key to getting the most out of training via effort (HRM) is understanding your heart is unique. There are a lot of variables that affect heart rate including your size, gender, age, and fitness level, as well as altitude and medications. An unfit heart will beat faster than a fit one; women's hearts tend to be smaller and have higher heart rates than men?
The best strategy for learning to use your monitor with purpose is to keep it simple.
Start to wear it around the house and during your runs. Get familiar with how your heart rate elevates and declines and what makes it do so. Before you put meaning to the numbers, just be with the numbers first.
Next, you can begin to understand how the numbers correlate to your current fitness. That is, what do the numbers mean, and how do you use them to train at the right effort level that day?
In the past, we used to recommend performing a maximal or submaximal test to figure out your highest possible heart rate and develop training zones heart rate and develop training zones based on this number. And if you weren't able to do this test (it is not fun), then we would use a formula that would estimate your maximum heart rate. The problem with this formula is it can be off by quite a bit (20 beats) and it just isn't accurate the vast majority of the time. We've evolved to using better options.
The current method is to find your lactate threshold (redline), or the point at which you shift from using a higher percentage of oxygen to glycogen due to the demands of the higher intensity activity. Or, as I like to describe it, the point at which you begin to ask yourself, "When is this going to end?"
Once you find the redline (which shifts with fitness) base your training zones (easy, moderate, hard) on this threshold and thereby begin to train and race based on what is going on metabolically (good stuff). Knowing where that redline is will allow you to train below it for easy to moderate runs and at or above it for high intensity workouts. Plus, smart runners will learn to tune into their bodies while they train and get a sense of what each level feels like regardless of the pace.