They work by giving you a random email address that is available for a short time (ex. 10min) and an inbox for any messages you recieve during that time (ex. an email verification message from a Lemmy server)
I didn’t want to recommend a specific one, however Googling “temporary email” should give you several options to choose from.
Just don’t ever forget your password to said Lemmy instance because you won’t be able to reset it.
Also be aware that sometimes the decision to reset the password is not done on your side. Sometimes due to attacks or similar website owners might decide a forced reset and that might require the email.