A ‘flake’ is the term given to a girl who does not get back in touch with you after either the initial meeting or an exchange of messages.
Most often this will happen over text – you get her number and she neither replies to your text messages nor does she answer your phone calls. Sometimes a girl will text back and forth a few times over a few days but eventually the messages dry up and she ‘flakes’. Just because the girl responded to your initial few texts, doesn’t necessarily mean the number is solid and will lead to a date.
Another way that a girl might flake is by not turning up to meet you at a prearranged time – although this is not so common. The vast majority of girls will have flaked long before you have attempted to set up a date. A girl will most likely have made up her mind about whether she wants to see you again before you initiate contact after taking her number.