In more detail
Speeds are quoted in bits but files are measured in bytes, and a byte is 8 bits, so divide by 8: a 100 Mbps connection moves at most 12.5 megabytes per second. Bandwidth can differ by direction; many home lines download several times faster than they upload. The rate you actually get, called throughput, is lower than the rated figure because of protocol overhead (headers, acknowledgements), the slowest link on the path and other users sharing the line.
Why it matters
Comparing a plan in Mbps with a download shown in MB/s without dividing by 8 makes a connection look eight times slower than it is. Extra bandwidth speeds up large transfers and many simultaneous streams, but it does little for a laggy video call or game, which is usually a Latency problem.
Example
Downloading a 2 GB file (2,000 megabytes = 16,000 megabits) over a 100 Mbps line takes at least 16,000 ÷ 100 = 160 seconds, about 2 minutes 40 seconds. The Bandwidth Calculator works this out for any size and speed.
Common confusion
Bandwidth is capacity (how much per second), while Latency is delay (how long one round trip takes). A wide motorway with a long distance still takes time to drive.