I must apologise for my lateness as I was caught in the bad weather.

was 是 be 动词的过去式，表示"处于某种状态"。 caught 是动词 catch 的过去分词，catch 本意是"抓住、捕捉"，这里引申为"遭遇、碰上"。 was caught 是被动语态结构，表示"被恶劣天气困住/耽误"，即"遇上了恶劣天气"。