What exactly is a cookie?
Cookies = small text files for saving settings in your browser for a website
1st Party Cookie = the website's cookie, maybe to keep you logged in or keep items in your shopping cart
3rd Party Cookie = a cookie loaded through a data company integrated with a website you were on
Why are they important?
They allow advertisers to track and advertise to specific users, with more relevant ads for better campaign performance.
Mobile apps don't usually support cookies, and when they do, they are housed in a walled sandbox unable to be shared with other applications or browsers. Mobile browsers, on the other hand, are much like any browser on your full-size computer and usually support cookies. This gap between the two major mobile viewing environments is the primary reason for frustration for digital advertisers. If you abandon your shopping cart in your mobile browser, I can't effectively retarget you in your favorite mobile app.
To compound the issue, certain browsers like Mobile Safari default block 3rd party cookies. This hasn't kept publishers from prompting users to re-enable 3rd party cookies, but has created a barrier for retargeting advertisers.
Given all of those issues, you would believe that advertisers would have given up entirely on using cookies for mobile, but they haven't. Mobile web accounts for >40% of traffic for many websites, and ~45% of that will be Android-based, which can easily utilize 3rd party cookies. Given that significant population of mobile users can still be targeted, ad networks and DSPs have continued using cookies to target and track users.
As the industry tries to bridge the gap between mobile web and app traffic, they have begun to incorporate a more universal form of user identification. Virtually every DSP, DMP, and ad exchange has embarked on a project to create a universal user ID that will allow them to sync cookies, device IDs, IP address, and unique identifiers to create a holistic view on a user across all of their screens and devices.
The likelihood of any one of these user databases becoming a standard is slim, suggesting that the future will look much like the past: platforms will do a user sync in the same way they previously did a cookie sync. The web cookie doesn't go away in this new model, but makes up one identification point out of many. These all-encompassing user databases should provide a better way for advertisers to market to users, but concerns over privacy may still shape the next evolution of user targeting.