When you go to a web site, you enter the domain name--
www.google.com. That resolves to an IP address like 128.33.12.129.
That's currently done using using DNS-- domain name service. Your internet-capable device talks to a DNS server and says "hey, where is
www.google.com at?" If your DNS server knows the answer, it will send it back to you. If not, it will check at a higher level to see who *does* know the answer, and go get it.
DNS didn't exist when the internet first got started-- there were so few machines that it was possible to manually track them all. This was done with what's know as a host file-- instead of going to a DNS server to see where
www.google.com is at, you would check that file to get the IP address.
The upshot of all this is that all tcp-IP based devices still use that host file as their first lookup point for addresses.
There are a variety of reasons why you might do this, but ad blocking is the most common-- I can say "ads.doubleclick.com" is really the same IP address as Google, so when it tries to get an ad, the program isn't there, and the ad never appears.
On windows, the file is in c:\windows\system32\drivers\etc\hosts. Not sure where it is on the Pre, but it should be relatively easy to find & update.
The hosts files I was seeing online were huge-- around 1.5MB-- which might be a lot for the Pre. What I would do is hit your favorite web pages with a real PC, see where the ads are coming from, then block just those sites.