Tracking pixels are like little beacons that you can use to see if someone has landed on your webpage from another website. They are great for conversion tracking / goal tracking where you can measure the success of your marketing efforts.
They are also really handy with remarketing / targeted advertising because now that the user has landed on your website, you can now send particular advertisements to that user, reminding them to come back to your website or take you up on your offer.
You will need a tracking pixel / website tag installed for each advertising channel you are looking to invest in.
Common tracking pixels
Google Tag Manager
You can use Google Tag Manager to easily add your tracking Pixels and Event Codes. This saves having to get your web developer to install the new tracking pixel or event codes. You simply do it all through Google Tag Manager. You will need your web developer to install Google Tag Manager on your website before they will work.