The word "express" means something similar to "show" or "tell". You can express an idea or a feeling:
She's never expressed a desire to study art before. I don't know why she's suddenly so into it.
I'm having trouble expressing exactly what I want to say.
But, as you can see from the example at top, you can also express yourself. This means to show your personality and what you're thinking.
Of course you don't feel comfortable expressing yourself freely at work. No one does!