We use defining relative clauses to give information about the person or thing that we are talking about, it allows us to identify them.

To form a defining relative clause, we use a relative pronoun and a verb.
eg. The boy, who doesn’t know how to swim, fell into the lake.

We can all reduce a relative clause by using a participle instead of a relative pronoun (who, that, which, …)
eg. The boy, not knowing how to swim, fell into the lake.

Examples :
– The boy who walks to school is my cousin. (relative clause)
– The boy walking to school is my cousin. (reduced relative clause)