[AがあるB - B that has A・B with A] Example: [障害がある]人 - A person [that has a disability]・A person [with disability]・A [disabled] person
The phrase [Aがある] modifies (qualifies) the noun B. [Aがある] therefore becomes a RELATIVE CLAUSE (describes the noun), similar to adjectives creating one bigger noun. Since in Japanese there are no relative pronouns (that, which, etc.), the phrase directly precedes the noun that is modified.
Relative clauses have some rules: 1. The topic marker は cannot be used 2. The subject marker が can be replaced with の (this marks a relative clause)