A sync folder shares each profile's mod list, sources, settings and mod config files between your computers through a folder another tool carries (Syncthing, Dropbox, a NAS). Mod files are never copied; each computer downloads them from their sources.

On each computer open Settings › Mods and profiles, find Sync folder, choose Choose… and pick the folder your sync tool keeps in step. Each game syncs into its own subfolder; a game's Profile sync setting can give it a folder of its own or turn sync off for it.
Work as usual; a few seconds after a profile stops changing, Mortar writes it into the folder.
On another computer a toast says <profile> was changed on <computer>. Review opens Changes from your other machines: Show diff, Apply (make your profile match) or Keep mine (overwrite the other revision).
If both computers changed the same profile, Mortar never merges: pick Use theirs or Keep mine.
To keep one mod enabled or disabled, or one config file, different on this computer, right-click the mod and choose Enable or disable on this computer only, or press Only on this computer in the mod's config editor. That choice is never written to the sync folder, and Apply and Keep mine leave it alone. Press the same item again to let it follow the sync. A profile must have synced once first.

Disable beside the folder stops syncing and leaves its files alone.