WordPress without ACF is a blog engine. WordPress with ACF is a CMS that can handle virtually any data structure. That's not an exaggeration. We've built real estate sites, booking systems, and product catalogs where the entire information architecture relies on ACF fields.
What ACF actually does
It lets you create custom fields and attach them to specific content types, taxonomies, or pages. Text fields, image galleries, relationships between posts, repeatable field groups, flexible layout sections. Everything is configured in admin without writing code (but the code is there if you want it).
Free vs Pro
The free version gives you basic fields: text, image, file, true/false, select, date picker. Enough for simpler projects. Pro (49 AUD per year) adds repeater fields, flexible content, and gallery. In most client projects we do, Pro is necessary.
Practical example: real estate site
Custom post type "Property". Field group "Property Details" with fields for price, living area, number of rooms, floor, year built, image gallery, and Google Maps. The client fills in structured data in form fields instead of trying to format information in a text editor. The result is consistent and searchable.
A tip we wish we'd gotten earlier
Export field definitions as PHP and put them in your theme or plugin. Then they're version-controlled with Git instead of only living in the database. If the database crashes or you need to set up a new environment, you won't have to reconfigure everything manually.
ACF Pro is now owned by WP Engine. Updates have become more frequent, and block editor compatibility has improved significantly since version 6.