Appeal is a global value that determines how appealing to shoppers the player's mall is. It is totalled throughout the entire mall and has no effect at individual level (structures with higher appeal will have the same odds of attracting shoppers as low-level ones, though high appeal decorations do seem to get photographed much more often).
Appeal adds shoppers in an exponential fashion (which in turn makes appeal gains logarithmic; early players will notice it only takes about 2500 Appeal to reach half the cap, while endgame players will need about 25000 to fully cap), up to a cap of 300 extra shoppers. Coupled with investments (two extra shoppers for each unique customer, and every player starts with 2 unique customers), this makes the highest possible amount of shoppers 452 at level 80.
The cheapest way to increase appeal is by using items wisely. Building the cheapest tiles also helps get a lot of early Appeal on top of earning a lot of extra coins from the individual bonuses, but make sure you have enough shops and attractions to make up for the amount of visits - or at least keep shoppers busy with decorations they can take pictures of - or your mall will quickly get overcrowded!