What you type in:
line1
<p>
line2
What you see on your page:
line1
line2
As you can see, there is a complete line of space between line1 and line2.
What you type in:
line1
<br>
line2
What you see on your page:
line1
line2
As you can see, there is no space between line1 and line2. By including the <br> tag, line2 is placed on the line right below line1.
What you type in (this is three in a row):
line1
line2
What you see on your page:
line1 line2
As you can notice, we used three tags in a row. This is because each tag creates only one space.