This is straight off the Apple site:
You can set Safari to prevent webpages from displaying pop-up windows or loading time-consuming content that may be unnecessary.
Unfortunately, many beneficial features use the same technology as ads and other content you might not want to see. So if you block pop-up windows or other web content, you might also block the animation and interactivity that make webpages interesting.
Block pop-ups on webpages
Open Safari if it is not already open.
Choose Safari > Preferences, and then click Security.
Turn on “Block pop-up windows.”
Blocking pop-up windows stops only those windows that appear when you open or close a page. It does not block pop-up windows that open when you click a link.
If you block pop-up windows, you might miss important information for a webpage. For example, a website that provides services to you might use a pop-up window to request your user name.
Block unnecessary webpage content
Open Safari if it is not already open.
Choose Safari > Preferences, and then click Security.
In the “Web content” section, select or deselect types of content you want to block or enable.
For an explanation of the options, click the Help button (looks like a question mark).