It Chapter Two's director, Andy Muschietti, promised there would be a lot of Easter eggs in the film. And there are - spoiler alert! - like a doll of Tim Curry's original Pennywise from the TV version of It and references to the Stanley Kubrick film of Stephen King's The Shining (which, amusingly, King didn't much like). There's a creature bearing a strong resemblance to one of the manifestations of The Thing in the 1982 movie and a character's response to seeing it - "You've got to be f---ing kidding!" - is also the response in the earlier film). And there's a cameo by the director in the pharmacy.
The term "Easter egg" seems to have originated in the computer-game world in 1979 when Atari employee Warren Robinett, uncredited for programming the game Adventure, hid his name in a specific place in the game that would only be revealed if triggered. The idea spread and hearing about "Easter eggs" can certainly encourage repeat playing (and movie viewing) although in the internet age they're usually easy to find online.
A story goes that there was a real Easter egg hunt on the set of The Rocky Horror Picture Show (1975) and some were not found (or maybe deliberately left?) and are visible in the movie though this seems to be retrospective rather than an origin tale. What exactly is an Easter egg in a movie is open to debate. It's generally considered to be an image, character or message that has to be looked for for but this is sometimes include references, homages, in-jokes, cameos and other elements. Some are more esoteric than others.
There are often Hidden Mickeys in Disney films and brief appearances by characters from other movies. Oddly, there's also an appearance in The Little Mermaid by the title fish character from the 1964 movie The Incredible Mr Limpet. - from Warner Bros. Well known is the the recurring Pizza Planet truck in Pixar movies, which often also contain callbacks to their earlier films.
In It: Chapter Two, a one-scene role for King is arguably not an Easter egg (it's far from fleeting) but a brief appearance by Peter Bogdanovich might be, though how many people would recognise the director of 1970s hits like Paper Moon is questionable (Muschietti is a fan).
More clearly Easter eggs are the appearance of R2-D2 and C-3PO among the hieroglyphics on a wall in Raiders of the Lost Ark and the plane markings OB-CPO on the aeroplane at the beginning.
Alfred Hitchcock's cameos in many of his movies - including a clockwinder in Rear Window, and a man in a newspaper ad in Lifeboat - might constitute one of the oldest and longest-running series of Easter eggs - later in his career he tended to appear early so the audience looking out for him would then focus on the film. And director John Landis's frequent insertion of the line "See you next Wednesday" from 2001: A Space Odyssey into his movies - on signs and posters, even in dialogue - is another recurring Easter egg to seek.
Cinephile director Martin Scorsese paid homage in The Departed (2006) to a motif in the 1932 gangster film Scarface by putting "X" - with varying degrees of subtlety - on screen to signify that characters were going to die.
There are many more to look out for and whether you want to use the resources of the internet as a guide or simply pay attention when viewing is up to you.