What do the columns mean in the downloaded raw data?

These vary according to the type of sensor in the row being examined - and all are cumulative timings:

HTTP sensor:
The five meta columns show the cumulative time after:
1. DNS lookup completed
2. TCP connection established
3. HTTP request sent
4. Server execution complete (i.e. first byte received)
5. Transfer complete, session finished.

These parameters are also shown in our performance graphs.

Ping sensor:
1. Packet Loss
2. Average Response Time

SMTP sensor:
1. Time to establish TCP connection
2. Total time (only if an email address was verified)

POP3 sensor:
1. Time to establish TCP connection
2. Total time (only if a username / password was used to login)

FTP sensor:
1. Time to establish TCP connection
2. Total time (only if a username / password was used to login)

DNS sensor:
1. Total time to request & receive DNS response

IMAP sensor:
1. Time to establish TCP connection

TCP sensor:
1. Time to establish TCP connection