on 4chan, every post is identified by a number, and "dubs" and "trips" means that the poster has gotten a number that ends in the same two digits (dubs, or doubles) or three digits (trips, or triples).

The joke is that the OP asked Anon if he said he'd roll dubs, and the replying Anon said he'd instead roll trips, which he in fact did, given that his post number ends in 555.

Furthermore, since incremental integer numbers are an obvious and easy way to assign identifiers to things such as posts on the Internet, they are used on many sites, so you can get dubs on more than just 4chan.

For instance, each comment on KYM also gets its own number (click on the "about X hours ago" below the username and look at the URL), and your very comment that I am now replying to gut number 5881811.

So, checked.