Viruses commonly change a little bit over time. This is common and expected. How much they change and how frequently they change depends a lot on how quickly the virus is spreading from person-to-person. The more quickly a virus spreads, the more the virus has a chance to change. When the virus changes, it may be called a new variant.
Variants are expected. The best way to slow the emergence of new variants is to reduce the spread of infection by taking measures to protect yourself, including getting a COVID-19 vaccine when available.