There are 3 meaningful ways we can interpret this number:
3.4 megabits per second
How communication devices are rated. Kilo means 1,000 and mega means 1,000,000. Examples include 56k modem and 10Mbit Ethernet
409.6 kilobytes per second
The way data is measured on your hard drive and how FTP programs measure transfer speeds. Kilo is 1,024 and mega is 1,048,576.
1MB file download
The time it would take you to download a 1 megabyte file at this speed.