what does text decoration mean in css code example

Example 1: text-decoration css

h1 {
  text-decoration: overline;
}

h2 {
  text-decoration: line-through;
}

h3 {
  text-decoration: underline;
}

h4 {
  text-decoration: underline overline;
}

Example 2: text-decoration

a{
    text-decoration:underline; // default in A tag
    text-decoration:none;
    text-decoration: overline;
    text-decoration: line-through;
    text-decoration: underline overline;
}

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