From the Centers for Disease Control and Prevention (official):
V-safe After Vaccination Health Checker
Use your smartphone to tell CDC about any side effects after getting the COVID-19 vaccine. You’ll also get reminders if you need a second vaccine dose.
V-safe is a smartphone-based tool that uses text messaging and web surveys to provide personalized health check-ins after you receive a COVID-19 vaccination. Through v-safe, you can quickly tell CDC if you have any side effects from the COVID-19 vaccine. This information helps CDC monitor the safety of COVID-19 vaccines in near real time. Depending on your answers, someone from CDC may call to check on you and get more information.
Register: You will need your smartphone and information about the COVID-19 vaccine you received. This information can be found on the vaccination record card you received during your vaccination; if you cannot find your card, please contact your healthcare provider.