I could do the test, but didn't understand some things

Tell us what’s happening:
Hey everyone,

I don’t understand why using left: -10000
width: 1px and height 1px. Also didn’t understand the use of overflow property.
Your code so far


<head>
<style>
.sr-only {
  position:absolute;
  left: -10000px;
  width:1px;
  height:1px;
  top: auto;
  overflow: hidden;
}
</style>
</head>
<body>
<header>
  <h1>Training</h1>
  <nav>
    <ul>
      <li><a href="#stealth">Stealth &amp; Agility</a></li>
      <li><a href="#combat">Combat</a></li>
      <li><a href="#weapons">Weapons</a></li>
    </ul>
  </nav>
</header>
<section>
  <h2>Master Camper Cat's Beginner Three Week Training Program</h2>
  <figure>
    <!-- Stacked bar chart of weekly training -->
    <p>[Stacked bar chart]</p>
    <br />
    <figcaption>Breakdown per week of time to spend training in stealth, combat, and weapons.</figcaption>
  </figure>
  <table class="sr-only">
    <caption>Hours of Weekly Training in Stealth, Combat, and Weapons</caption>
    <thead>
      <tr>
        <th></th>
        <th scope="col">Stealth &amp; Agility</th>
        <th scope="col">Combat</th>
        <th scope="col">Weapons</th>
        <th scope="col">Total</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <th scope="row">Week One</th>
        <td>3</td>
        <td>5</td>
        <td>2</td>
        <td>10</td>
      </tr>
      <tr>
        <th scope="row">Week Two</th>
        <td>4</td>
        <td>5</td>
        <td>3</td>
        <td>12</td>
      </tr>
      <tr>
        <th scope="row">Week Three</th>
        <td>4</td>
        <td>6</td>
        <td>3</td>
        <td>13</td>
      </tr>
    </tbody>
  </table>
</section>
<section id="stealth">
  <h2>Stealth &amp; Agility Training</h2>
  <article><h3>Climb foliage quickly using a minimum spanning tree approach</h3></article>
  <article><h3>No training is NP-complete without parkour</h3></article>
</section>
<section id="combat">
  <h2>Combat Training</h2>
  <article><h3>Dispatch multiple enemies with multithreaded tactics</h3></article>
  <article><h3>Goodbye, world: 5 proven ways to knock out an opponent</h3></article>
</section>
<section id="weapons">
  <h2>Weapons Training</h2>
  <article><h3>Swords: the best tool to literally divide and conquer</h3></article>
  <article><h3>Breadth-first or depth-first in multi-weapon training?</h3></article>
</section>
<footer>&copy; 2018 Camper Cat</footer>
</body>
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Challenge: Make Elements Only Visible to a Screen Reader by Using Custom CSS

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