Linking listings tells Host Tools that a property you have on multiple channels is the same listing. For example, if you have an apartment listed on Airbnb, VRBO, and Houfy you will need to link those three listings so that Host Tools recognizes them as one listing, not three separate listings.
It is important what order you link listings. The listing whose settings you go into will be the "parent" listing. Host Tools will use the "parent" listing's message and pricing rules. Any smart locks you have for that listing will also need to be linked to the "parent" listing.
Make sure you choose the listing you want to be the "parent" listing and then link listings to that, instead of the other way around. You can always unlink them, and then re-link them in another way if you change your mind.
Click on your profile photo in the upper right-hand corner.
Click on "Settings".
Scroll down to the listing you want to be the parent and click the "Settings" button next to it.
Under the heading "Link Listings" choose the listing you'd like to link to it (as a child) from the drop-down "Listings" menu.
If you would like Host Tools to automatically block your calendar on one channel when you get a reservation on a different channel then you will also need to enable availability syncing.