Consider these sentences:
A woman is talking to Martin.
Our new teacher is a woman.
We can combine these sentences using relative clauses:
Our new teacher is the woman who is talking to Martin. OR
The woman who is talking to Martin is our new teacher.
When we use this structure, we use pronouns, relative pronouns, such as WHO in the example.
If you need to learn more about Relative clauses, you can go to Perfect English Grammar.
You can practise this point with the following exercise: