When someone substantiates something, they provide evidence to support or prove it.
Substantiating something is important because it makes it believable or true.
Examples:
She substantiates her claim with strong evidence.
The data substantiates the theory.
The scientist substantiates her findings with detailed research.
He substantiates his story with video proof.
The witness's testimony substantiates the defendant's alibi.
The photograph substantiates the existence of the mysterious creature.