The goal of segmenting is to hyper-personalize your communication so it's always relevant and adds value. 

Let's start with your full list, we'll call this "All Subscribers"—this is your whole pie.
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You can manually slice this list, like in this example:
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Let's say you'd like to group your existing clients. You'll want to cut a slice from that pie and name this segment "Clients".
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You can do this by uploading a CSV spreadsheet with your Clients, or you can search for each client in your "All Subscribers" view (Audience > All Subscribers), and use the bulk selection and action to add them to this new segment.
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The result: now you can send a very personalized email to this segment that says "Hi [name], because you are my client, I wanted to share this with you."
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Or you can automatically slice this list, like in this example:
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Let's say you want to grow your list (duh, that's why we're here, right?). What you'll want to do is put a form on your website so new subscribers can opt in to receive your content.
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Let's call this content "Top 10 Business Tips". When you create your Lead Magnet form, you'll want to indicate that these new subscribers get added to your "Top 10 Business Tips Opt-in" segment.
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These new subscribers will still be part of your whole pie ("All subscribers"), but they're sliced as your "Top 10 Business Tips Opt-in" segment.
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The result: now you can automatically deliver your "Top 10 Tips" email, and even automate a workflows a few days later to send a hyper-relevant, hyper-personalized email that says "Hi [name]! Because you downloaded my Top 10 Business Tips, I thought you'd like my Entrepreneurship 101 Guide too".

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