Refers to the peak commuting time when trains and roads are crowded. Often used in compounds like ラッシュ時の電車.
ラッシュ時の電車はとても混んでいる。
Trains during rush hour are very crowded.
ラッシュ
I commute avoiding rush hour.
Wasei-eigo compound of ラッシュ (from English 'rush') and 時 (じ, 'time').