expression
a matter of course; what is to be expected; par for the course
Used to state that something is natural, obvious, or to be expected under the circumstances. Often appears in explanatory or evaluative contexts.
彼が怒るのは当然のことだ。
It's only natural that he gets angry.
努力した人が成功するのは当然のことです。
It's a matter of course that those who make an effort succeed.
当たり前 is an adjective meaning 'obvious' or 'natural', while 当然のこと is a noun phrase meaning 'a matter of course' or 'something to be expected'.
Phrase composed of 当然 (touzen, 'natural' or 'obvious') + の (possessive particle) + こと (koto, 'thing' or 'matter'). Literally 'a matter of naturalness'.