DNA always comes as DNA strands.

The DNA strands are DNA from old football stars from year 2100-3000.

The DNA you find can be used to inject into robots (from your squad) and hereby build a new player.

You can also once a player is build use DNA strands to upgrade or even overclock your players.

DNA cannot be used to anything besides injecting into players (besides selling/scrapping the DNA for cash)

NOTE that you can collect DNAs and get rewards. Check the discovery section for which DNAs you have collected, what you are missing to collect and what reward you can earn.