The Kripke era refers to the seasons of Supernatural helmed by its creator, Eric Kripke. It encompasses Season 1 up to Season 5. The first five seasons contain the exposition of the main characters of the show, and, although contained season-long story arcs, actually built up to a climactic five-season arc about supernatural forces working to usher in the Apocalypse. Kripke himself originally named the plan for his show the "five-year plan", covering the first five seasons of the show, all leading up to the events in Swan Song. The show was supposed to end at this point but Kripke handed over the role as showrunner to Sera Gamble for Seasons 6 and 7.
The plot of this era revolves around a conflict between the angels led by the archangel Michael, and the demons of the fallen archangel Lucifer, and their respective predestined vessels, Sam and Dean Winchester.
Azazel, a powerful yellow-eyed demon sought to release Lucifer from his cage, and orchestrated a complex plan that will create possible vessels that will host Lucifer when he is freed from the cage. His plan also simultaneously allowed the release of an army of demons onto earth to prepare for the war, and also to secure a righteous man that will spill blood in hell, which begins the many steps leading to the eventual escape of Lucifer.
At this point, the angels were sent down to Earth in an attempt to stop the release of the fallen archangel Lucifer. However, the highest members in command of the heavenly host actually plan to let Lucifer be set free so they could usher in the Apocalypse and, assuming that the Angels won the war, paradise would follow. Angels of lower commands were not privy to this, and some of the angels actually worked with the plan to release Lucifer.
The Era ended with the apocalypse being averted by Sam, Dean, Castiel and Bobby Singer, and with the leaders of the two warring factions, also the two most powerful Archangels in existence, trapped and basically incapacitated inside Lucifer's cage.