Fixed-length format generally means that it's not comma-seperated, but that the string is well-defined. For example, if each record consisted of a customer id number (up to 6 characters), first name (up to 20 characters), surname (up to 20 characters) and phone number (up to 12 characters), then each record would be a string of 58 characters followed by a line break, and you'd know that the first 6 charcters always constituted the customer number, the next 20 the first name etc.
Exactly how it's written varies. Elements will sometimes need to be "padded" (e.g. my first name is only 4 characters, so the other 16 must be filled with something - commonly spaces, but they could use something else).
So examples might be
As for importing it to a database, because you know where each element starts and ends it will be very simple to write a script or whatever to insert commas in the appropriate places and create a CSV file. Elements can be trimmed or whatever to remove any padding characters depending on what they use.
12 Nick Wilson 555-5555
Hmm, not a very coherent reply but I hope you get the gist .