If two things are alike, they resemble or are similar to each other. You and your sister might be alike in many ways, including your love for strawberry ice cream.
If all the houses in your neighborhood look alike, it's hard to tell one from another, and if their parents always dress twin brothers alike, it's going to be impossible for people to remember who's who. Things that are alike are the same. The original form of the word was aliche, from the Old English gelic, "similar," with a Germanic root, which it shares with the word like.