To pick someone's brains is a common English expression that we use when we want to get information from someone about a certain topic.
More often than not, someone will know more about a subject than you. You will need to ask that person to give you the information you need.
The meaning is to ask a person for advice about a subject that they have knowledge about.
Examples:
Can you help me with this problem? Let me pick you brain.
John will know the answer. I am going to pick his brain.
Here's another similar lesson for you to practice more listening.
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I think, need more listening.