To miss or fail to do something (often used in negative forms). It can mean to omit or not perform something one should do.
彼は健康のために運動を欠かさない。
He never misses his exercise for health.
重要な手順を欠かしてはいけないので、丁寧にチェックした。
Because you must not omit important steps, I checked carefully.
うっかりして報告を欠かしてしまい、問題になった。
I carelessly failed to submit the report and it caused a problem.
To do without; used mainly in negative constructions to express that something is indispensable (as in 欠かせない). While it can mean 'to go without', in practice it's often seen in negative forms meaning 'cannot be omitted'.
この薬は私にとって欠かせないものだ。
This medicine is indispensable to me.
栄養バランスの良い食事は健康維持に欠かせない。
A well-balanced diet is essential for maintaining health.
忙しくても彼女は日記を書くことを欠かしたことがない。
Even when busy, she has never failed to write in her diary.