Typically the documentation is really worth to read. Lets start with
https://robotframework.org/SeleniumLibrary/SeleniumLibrary.html#Handle%20Alert.
Actions are defined to be "DISMISS", "ACCEPT" and "LEAVE". Then, there are examples:
Examples:
Handle Alert # Accept alert.
Handle Alert action=DISMISS # Dismiss alert.
Handle Alert timeout=10 s # Use custom timeout and accept alert.
Handle Alert DISMISS 1 min # Use custom timeout and dismiss alert.
${message} = Handle Alert # Accept alert and get its message.
${message} = Handle Alert LEAVE # Leave alert open and get its message.
Last example in the official documentation.
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