Drop rate is a pretty simple statistic, which they calculate by adding up the amount of catchable balls (catches + drops) thrown a guy's way, divide it by his number of drops and multiply by 100.
The idea is that simply looking at the total number of drops a guy has doesn't tell the whole story. If he's getting more passes thrown his way, chances are he'll have more drops. So drop rate tells us how often guys are dropping passes.
For instance, Victor Cruz & Donnie Avery both dropped 12 passes this season, tied for the 4th most in the league. The difference is that Cruz had 98 catchable balls thrown his way while Avery only has 72. So Cruz's drop rate is 12.24 and Avery's is a league worst 16.67. The salsa man's figure isn't good, but its not among the 10 worst either.
As for guys whose hands were impressive last year, the Eagles have 2 of the top 3. #1 in the league is Jason Avant, who was not credited with a single drop on any of the 53 catchable balls thrown his way.
The other good hands guy is none other than DeSean Jackson, who finished with a drop rate of 2.13, good for third best in the NFL. PFF credited him with just one drop this season. It's a major reversal for DeSean, who was previously one of the worst in the league in terms of drop rate.
The numbers back up what we saw from DeSean this past year. Prior to his injury, he was on pace for career highs in catches and yards and really looked like a guy who had fully rededicated himself after getting a big contract. Here's hoping the trend continues next season.