しかし(but; however) is a conjunction which is placed between two sentences. It is used at the beginning of a sentence to present an idea which is in contrast with the one expressed in the preceding sentence, or to present something which differs from the expectation based on what is stated in the previous sentence.
The Shinkansen[Bullet Train] is fast. However, it is expensive.
なつやすみは ながいです。しかし、ふゆやすみは みじかいです。
Summer vacation is long. However, winter vacation is short.