-F (Fast (limited port) scan) Specifies that you wish to scan fewer ports than the default. Normally Nmap scans the most common 1,000 ports for each scanned protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in order to know which ports are the most common (see the section called “Well Known Port List: nmap-services” for more about port frequencies). If port frequency information isn't available, perhaps because of the use of a custom nmap-services file, Nmap scans all named ports plus ports 1-1024. In that case, -F means to scan only ports that are named in the services file.