On the buildd db_status page, what do the columns mean?

Well, this is obvious, isn't it? ;-)

This too. It's the shortname of the buildd. Usually this can be found as well in the build logs you can find on buildd.debian.org.

Lastseen is the timestamp when the buildd updated its status the last
time. The time is in localtime, which is for the server location during
the northern summertime CEST = UTC+2 and during winter CET = UTC+1.
Usually all times on this site are in that timezone if not otherwise stated.

The interval is the time between two updates and is important for the last column.

expected at
This is probably the most important column. It defines until when a buildd have to update its status again to not marked as being down. It is 4x the interval duration. After 1.25x the interval the machine will be marked as "no response".