The line is spoken after the witches have been chanting a spell. They are using their magic to create a storm, and they are about to meet Macbeth, who is on his way to become King of Scotland. The line suggests that something evil is about to happen, and that Macbeth is going to be involved.
The phrase "something wicked" can be interpreted in many ways. It could refer to Macbeth himself, who is a ruthless and ambitious man who will stop at nothing to get what he wants. It could also refer to the witches, who are evil and dangerous creatures. Or, it could refer to the storm that the witches are creating, which is a sign of chaos and destruction.
Whatever the interpretation, the line "By the pricking of my thumb, something wicked this way comes" is a powerful and evocative piece of writing. It creates a sense of suspense and foreboding, and it sets the stage for the events that are to follow.