These are full-page advertisements that Spielberg and Lucas purchased in Variety magazine as open letters to each other, congratulating each other on their movie success.
The one above is from the December 2nd, 1977 edition. It is from Spielberg to Lucas congratulating him on Star Wars beating Jaws in domestic video rental charts.
When ET broke that record in the early 80s, Lucas took out the ad below to congratulate Spielberg.
In 1997, when the re-release of the Star Wars broke the ET record, another open letter was published showing ET crowning R2D2.
There is apparently another one, which I was unable to find, that both Lucas and Spielberg wrote to James Cameron when the movie Titanic broke all of their records.