Oneself — referring to oneself as a noun (somewhat literary or formal).
自らの力で問題を解決しなければならない。
You must solve the problem by your own effort.
彼は自ら志願してその任務を引き受けた。
He volunteered himself to take on that mission.
自らの過ちを認めるのは大切だ。
Admitting one's own mistakes is important.
Personally; by oneself — an adverb meaning to do something oneself or in person.
社長自ら社員に説明を行った。
The president himself explained to the employees.
彼女は結果を確かめるために自ら実験室に入った。
She herself entered the laboratory to check the results.
問題の現場へ自ら足を運ぶべきだ。
You should go to the scene of the problem yourself.