It's not such a tricky concept. The verb "jsem" already contains the information that it's "I" doing the "being", making the "I" redundant. In English, "am" also works only with "I", but most verbs have the same form in all persons/numbers except for the 3rd singular (he goes) and some don't even have this exception (he can). Therefore, you need to include the pronoun in English (I go, you go, we go, they go) to know who is doing the action.
In Czech (and Spanish and many other languages), you nearly always know who is doing the action from the verb alone. E.g. "mám" (I have), "máš" (you have), "máme" (we have), etc. The pronoun becomes redundant. Including it in the sentence means stressing it in a way:
- Jsem žena. - I'm a woman.
- Já jsem žena. - I am a woman. As for me, I'm a woman.
- Mám dítě. - I have a child.
- Já mám dítě. - As for me, I have a child.
- Dítě mám já. - It is me who has a child.