The units
A bit is the smallest unit of data. Eight bits make a byte, which can hold one of 256 values, enough for one letter in a simple text. Lowercase b means bit and uppercase B means byte: Mbps is megabits per second, MB is megabytes.
Converting
Divide bits by 8 to get bytes. A 100 Mbps line is 12.5 MB per second, so a 2 GB file takes at least 160 seconds, about 2 minutes 40 seconds. The Bandwidth Calculator does the sum and the Data Storage Converter converts sizes.
Common misunderstanding
Advertisers use bits because the numbers look eight times bigger. Check the capital letter. See Megabits vs Megabytes for more.