Serious. An infection in the foot can be a serious problem. If untreated it can cause tissue destruction and may lead to amputation or rarely death.
Untreated cut. Any wound/cut has the potential for infection, and that can lead to further issues, such as sepsis (blood infection) and abscesses (boils). Most cuts will heal without infection, especially with basic first aid, but if there are signs of infection, such as redness, pain pus, fever or swelling, you should see your doctor. Getting a tetanus booster is also recommended.
Worse infection. Any infection can get out of control and cause other more sever issues. Have it evaluated and treated to avoid complications.