What is the "meta name = 'viewport'" for?

It is an HTML5 tag and is used to optimize mobile sites, this tag helps define the width, height and scale of the area used by the browser to display content. Its basic use is as follows:

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">

As additional information, I share a video where it explains much more: link

It is to configure the space that your website will use, as you have mentioned it is like this:

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">

the part that says:

width=device-width

defines the width of the website in this case the width is the width of the device. The other two that say:

initial-scale=1, maximum-scale=1

serve to define the scale of the website, have you realized that you can zoom? This is what the scale refers to when defining an initial scale and an equal maximum. You make sure that you can not zoom, this is useful in mobile web sites because zooming in on them usually spoils the visibility of the site.

If I remember correctly, it serves to control the composition for mobile phones, which helps us in the presentation of our web projects on mobile phones