The DJIA (Dow Jones Industrial Average) is an index of 30 companies that are picked to represent the broad economy. As this Fox Business article notes, Apple is replacing AT&T in the index. Given that Apple is the largest company by market cap, why is Apple just now entering the index? The answer is that the Dow is price-weighted, so Apple's recent 4:1 split brought the price low enough, such that it would not unduly influence the overall index movement.
Dolvin, Steven D., "Apple Added to the Dow" (2015). All Chapters. 130.