もし is an adverb used to emphasize the nuance of ‘if’, with expressions like と, ば, なら, たら, and ても. Literally, the meaning of もし is close to ‘while slight’, or ‘while small’.
To use もし, simply add it to the beginning of any phrase that uses one of the conditional expressions listed above.

• もし彼（かれ）を動物（どうぶつ）に例（たと）えると、猫（ねこ）に似（に）ています。
  If we were to compare him to an animal, he resembles a cat. (Even if you only slightly compared him)
• もし今日（きょう）来（こ）れれば来（き）てね。
  If you can come today, please do. (If, on the slight chance you can come)
• もしまだ頭（あたま）が痛（いた）いなら、医者（いしゃ）に行（い）った方（ほう）がいいよ。
  If your head still hurts, you should go to the doctors. (If, on the slight chance it does hurt)
• もし買（か）い物（もの）に行（い）ったら、お菓子（かし）を買（か）って来（き）て。
  If you go shopping, can you buy some snacks? (If, on the slight chance you do go)
• もし雨（あめ）が降（ふ）っても遊園地（ゆうえんち）に行（い）きます。
  Even if it rains, we will still go to the amusement park. (If, on the slight chance it does rain)