The Meaning of Building Rapport With Others
According to the Cambridge Dictionary, the definition of 'rapport' is a good understanding of someone and an ability to communicate well with them - see: https://dictionary.cambridge.org/dictionary/english/rapport
However, you may ask 'can I communicative well with someone even if I haven't established rapport with them?' and the answer is yes. You can.
We communicate with people on a daily basis, many of whom we either do not know at all or know very little about. But that doesn't mean to say that we cannot communicate well with them.
In the context of the workplace, the purpose of establishing rapport with someone is to get know them well. When you know a person well, you're in a better position to create a meaningful work relationship with them. You are also better able to tailor and adapt your communication style and approach with them.
To understand someone, you need to take a proactive and genuine interest in them. You do this by spending time with them, asking questions and seeking out ways to get to know them better.
Remember, having regular communication with someone is essential to maintaining a good working relationships and it may also help to prevent issues from blowing up. See 'The Importance Of Having A Direct, Ongoing And Transparent Relationship With Your Manager' by Matthew Coppola.