What does id do in html code example

Example 1: html id

<p id="newid">this is the id</p>

<style>
#newid {
  background-color: green;
  text-align: center;
}
</style>

Example 2: html id

<style>
#grepperHeader {
  background-color: lightblue;
  color: navy;
  padding: 40px;
  text-align: center;
}
</style>

<h1 id="grepperHeader">grepperHeader</h1>