Several motorists were snowed in on the mountain pass.

were 是 be 动词的过去式复数形式，表示被动语态或状态。 snowed 是动词 snow 的过去分词形式，snow 本意是"下雪"，在这里与 were 构成被动语态表示"被雪困住"。 in 是介词，表示"在...里面"，在这里表示被困在某个地方的状态。 were snowed in 是一个固定短语，表示"被大雪困住/围困"，通常指因大雪无法离开某个地方。