I am going to assume they mean you don't talk about your feelings. Maybe they also mean you don't talk about your deepest thoughts, either. Like you withhold the real essence of who you are. If this is what they mean, sometimes we don't emotionally open up to people out of fear they will judge, out of having stuffed or masked our own feelings so we don't even know what we actually feel, or what some used to call lack of emotional intelligence. Now I don't know you but am answering in a general way. When people tell us this stuff, we usually need to ask them to clarify. Sometimes they are right and sometimes they are nonsensical.

Big hug cc... I agree I would ask them to clarify as well.... Sometimes it can be a fear of trust and that can come from childhood

Yes I did hide how I feel most of the time. But sometimes I explain my deep thoughts on a matter but I'm very calculative about it and try my best timing.

That being said how do you express feelings productively and positively?

With honesty. I continue to find deeper levels of honesty with myself and that comes through to others in an increasingly positive way.
There are free online breath work sessions which are amazing for opening up your feelings to yourself. Also any talks or books by Thich Nhat Hahn are wonderful in increasing self love which is necessary for people to receive you warmly.
Big bird style….. love yourself so you can love the world (and communicate well)

Being able to open up to another person takes a lot of courage if you've been challenged with trust issues.

Just my thought, the first person to open up to is You. If you've blocked out emotions as protection, that needs to be worked on.

For my own journey in life, I've used therapy to help guide me in "peeling away each layer of an onion'. The reason for onion, well I needed to shed a lot of tears.

It's not an easy task to work on yourself, in the mean time think about the word 'empathy'.....try using that with yourself and with other people....be easy on yourself.

Caroline Myss has a lot of videos that could open you up.

Best Of Luck :)

I think trying to just open up emotionally is hard on people due to walls and boundaries you created for yourself due to past experiences, to protect yourself from getting hurt. What those experience are, I am not sure only you know. But it can stem down from a relationship with your parents, or a sibling, some other family member, maybe some close friends, exes, maybe a coworker or boss or neighbor, high school. You can have this boundary or wall up from one person or multiple.
I know for myself I am like this in certain situations and others not. Some people I am comfortable opening up to them, and others not.

Most people have filters or know that even if a fool keeps quiet he'll be thought wise. I have no filter and share deeply personal things to strangers, so no problem opening up here.