It may be that you aren't really sure what the term "boundaries" means.
A boundary is a limit you set, which reinforces your sense of self, and protects you from exploitation by others. All healthy relationships have boundaries. If, as a child, your boundaries were ignored- your feelings seen as unimportant, for example; the fact that you didn't feel comfortable giving Uncle Colin a kiss, but you were made to feel bad if you refused, or your family did not show how healthy boundaries work, then you would find boundaries hard to understand.
I have read a really good description of 4 types of boundaries that explain them really well- thanks to @the.holistic.psychologist
1. Physical boundaries: around our physical limits and personal space e.g.people touching us, allowing ourselves to rest, privacy.
2. Emotional boundaries: how you feel, who you engage with, how much of yourself you share.
3. Resource boundaries: around your time and energy. e.g. feeling able to refuse social invitations, agreeing to help for an agreed time only.
4. Material boundaries: about your possessions, how they are used and treated, e.g asking for someone to be sure to return a book they are borrowing.
It is common to feel guilty and anxious when you start to set boundaries. People with codependency issues especially find this hard. Having limits, and respecting the limits of others is not being unkind, but respectful of yourself and others. Boundaries show a sense of self-worth. You aren't responsible for how others react to your boundaries, only for how you respond to their reaction. It gets easier with practice.