To opt into your content being searchable, once your instance is upgraded to support this feature, head to the your profile, and the new “Privacy and reach" tag, then tick the "include public posts in search”.
We will try to ensure users know this setting exists, nothing 100% defined yet, but we may add a dismissible banner asking to review your privacy settings page.
Full-text search allows you to search for words in the content of statuses (if your instance has ElasticSearch configured), compared to the current version which only allows searching for hashtags.
In addition, you get a whole lot of search modifiers for advanced search (no UI for those yet):
has:image / has:video / has:audio / has:media
has:poll / has:link / has:embed
is:sensitive / is:reply
language:en
from:account
before:2023-08-08
after:2023-08-08
during:2023-08-08