Airbag compatible (sometimes called 'airbag ready') means the backpack is built with a dedicated sleeve and attachment points to accept a removable avalanche airbag module, but the module itself is sold separately. You get the pack first, then add the airbag system when you're ready—either right away or down the road when your budget or backcountry plans change.
The biggest advantage is modularity. If you own multiple compatible packs from the same brand, you can swap one airbag module between them instead of buying a separate integrated airbag pack for each setup. That said, the module does take up some internal pack volume, and you'll need to factor in the extra cost of purchasing it separately—typically several hundred dollars on top of the pack itself.
This option makes the most sense if you want flexibility to add an airbag later, or if you ride with different packs for different tours and want one module to work across them all.