Tooth "whitening" is defined as any process that will make teeth appear whiter
Either your hygienist or dentist takes a brush to paint the gel on your teeth. When they finish, they will switch on the light or laser and point it directly at the gel.
You sit under the light for around 20 minutes as the light transforms the oxygen in the gel and causes it to dive deep beneath your enamel to reach your color pigments.
Don't worry if the hygienist walks away for a minute. The light turns off automatically when it's finished.
After your 20 minutes is up, the hygienist removes the bleach from your teeth. They then paint on a new coat to prepare you for a second round.
It's likely you'll go through three to four 20 minute rounds throughout your treatment. The exact number depends on the system chosen, the strength of the gel, and the color of your teeth now compared to the shade you want.