Secondary emotions are the judgements of primary emotions. For example, I'm afraid I'll lose control when I get angry. Anger is the primary emotion and fear is the secondary emotion.
It's important to identify and be cognizant of secondary emotions because they can mask over primary emotions. This makes it difficult to express primary emotions in a healthy way.
Half way through a difficult week I'll often feel like I've made no progress on my long-term goals. I've been fighting fires for days and feel like I'm not learning and growing. I go to my one-on-one meeting with my manager and he asks, "how is it going"? I say things are terrible, our teams are too junior and don't know what they're doing. Our systems teams are slow and incapable. Our code is a mess and our site lacks reliability.
In this example I'm showing anger, disappointment, and frustration. But what is my primary emotion? Emotions can be difficult to identify, I recommend finding a good emotion wheel. In this case, when I dig deep down I find that the primary emotion is actually fear. Fear that I'm not making progress by learning and growing, that my role may not be what I thought and I may have to change jobs, that I'm not living up to the expectations in my role and I might get a bad review.
It might be true that our systems and teams are a mess, but expressing anger in the heat of the moment is entirely unproductive to my career. If instead I expressed my fear about my own career and the activities I've been doing in a healthy way, then I can shortcut potentially weeks or months of anger and frustration.
Let's replay the scenario, my manager asks, "how's it going"? I identify my primary and secondary emotions and take a big shortcut straight to what would be helpful for me. I explain how I feel lost, like I'm spinning my wheels. I share the mis-match in expectations for my role and open up about what helps me feel fulfilled and happy. We work together on a plan to incrementally move me in the right direction and my manager has gained critical insight into how to help me move quickly in my career.
Primary emotions are important indicators that we need to learn to express in a constructive, healthy, and professional way. Often this is through candid feedback and vulnerable conversation.
Being vulnerable is more difficult than giving in to a secondary emotion. We fall back on our secondary emotions because they take the difficult primary emotions and our responsibility to deal with them out of the picture.
However, you'll not only feel better as a self-fulfilled person by tackling primary emotions head-on, but you'll be accelerated through your career. Emotional maturity and the ability to give good feedback are hallmarks of senior engineers. This maturity is difficult to build, but easy to spot, and every great leader I've worked with has it.