For many common antibiotics, a small amount of alcohol will not stop the medicine working, though alcohol can worsen side effects like drowsiness or stomach upset and slow your recovery, so it is sensible to limit it while unwell.
There are important exceptions. With metronidazole and tinidazole, drinking alcohol can cause flushing, severe nausea, vomiting, and a fast heartbeat; you should avoid alcohol during the course and for a couple of days after. Some other antibiotics also interact.
Because the rules differ by drug, always check the patient leaflet or ask your pharmacist about your specific antibiotic before drinking.