Follow these steps to share a folder from your host macOS to guest Kali Linux running inside VirtualBox.
Step 1: From your macOS, Go to "System Preferences... >> Sharing"
Step 2: Enable the File Sharing option and add a folder to share
Step 3: Open VirtualBox >> Select Kali Linux (or any other image)
Step 4: Click on "Settings >> Share Folder"
Step 5: Click on Add new shared folder button and add the same folder you previously added in macOS from Step 2
Step 6: Log in to your Kali Linux image
Step 7: Open Terminal and type the command "sudo adduser $USER vboxsf"
Step 8: Reboot Kali Linux