Used when two or more events happen at the same time or in a similar way by chance, without any causal connection.
It's a coincidence that we have the same birthday.
His story and the evidence showed a coincidence.
偶然 is an adjective/noun meaning 'accidental' or 'by chance', while 偶然の一致 specifically refers to a 'coincidence' as a noun phrase.
一致 means 'agreement' or 'matching'; 偶然の一致 emphasizes that the matching is accidental.
Compound of 偶然 (accidental) + の (possessive particle) + 一致 (agreement). The phrase is a straightforward combination of its parts.