You can set pages as Protected which means they can only be viewed by logged in administrators and members. Whilst administrators can make changes to the Protected pages, members can only view them.
Adding Members to your Site
Firstly it's worth making clear that there are two different ways you can give people Member access to Protected pages on your site. In both cases you do so by going to Dashboard > Site Settings > Administrators and click Add Member.
Adding an Individual Member
You add individual members by setting each person up with their own login. This means that all members have their own password and if you need to you can remove a single member in future blocking their access.
It also means you can in future promote that member to (eg) a content editor, giving them some editing access.
The downside is that if you have 100 members you'll need to add them all one at a time, which is why you'll see an 'add General Member' link in the screenshot.
Adding a General Member
If you prefer you can add a single General Member account. This way you can create a single account with a single password which all your members use to log in. This is much easier for you to manage than a long list of members, but it doesn't give you the ability to lock one member out. If you ever need to change who has access you'll need to change the password for the General Member account and let all those you do still want to have access know the new password.
Whereas Individual Members can reset their own password if they forget it, a General member can't, so you have the option to add a note which will show on the login form letting your members know who to contact if they need help.
Once you've set up individual or general members they can log in by going to your Admin page login address. However you can also add a members-login-form block to any page allowing your members to log in from there, which looks a lot nicer. (Don't add a member-login-form block to a Protected page as they can't see it until they've logged in!)
What can Members see?
So what's the point of having website members? Well your admin system allows you to change the visibility of every page on your site (except your Home page). Once of the visibility settings is 'Protected'. This allows you to create a page which you (as administrator) can see but the public can't.
This can be useful when you're working on a page which isn't ready to go live yet, but it's also how you set a page which members can see but the public can't.
In the screenshot you can see a dropdown under Members Area with a number of pages in it. Each of them has been set to Protected and therefore they have square brackets around their names. These pages are only accessible and visible on the menu to someone who is logged in either as an administrator or member.
Note that in this case we've added the Protected pages to the menu below the Members Area page, but there's no requirement to do this. Protected pages can be added anywhere on your menu.