Used after the negative stem of a verb to indicate one has no choice but to do something; 'cannot help but ~' or 'must ~'. Formal/written expression (usually written ざるを得ない).
期限が迫っているので、残業せざるを得ない。
The deadline is approaching, so I have no choice but to work overtime.
電車が止まったので、歩いて帰らざるを得なかった。
The train stopped, so I had no choice but to walk home.
証拠が揃っているから、彼の主張を認めざるを得ない。
Since the evidence is complete, I have to acknowledge his claim.