Signing up gives you access to the Structurizr cloud service, with the paid feature set as described on the products page, for a trial period of two weeks. After this time you will switch to the free feature set, with the option to start a paid subscription.
You will need to verify your e-mail address to use your account, so please check for typos.
Signing up on behalf of an organisation? Consider using a shared/group e-mail address to ensure continuity of access when individuals leave your organisation, or are otherwise unavailable. See Help - Corporate accounts for more details.