A more thorough explanation:

Definition: The middle thread refers to the center line of something, especially an imaginary line drawn lengthwise through the middle of a stream's current.

Example: When fly fishing, it's important to cast your line into the middle thread of the stream to catch the most fish.

Explanation: The example illustrates how the middle thread is the center line of a stream's current. By casting your line into this imaginary line, you increase your chances of catching fish because it's where the current is strongest and where fish are most likely to be feeding.