The wifi-ap snap will try to automatically configure the access point after installation with the best options it can automatically determine. You can check with
$ wifi-ap.status
if the automatic configuration was successful and the access point is already active.
By default the automatic wizard, which runs when the wifi-ap snap is installed, will choose secure enough default password and will enable WPA2-PSK security. You can find the selected password when logged into the system the wifi-ap snap installed on by running the command:
$ sudo wifi-ap.config get wifi.security-passphrase
To enable the WiFi access point manually after the snap was installed and all plugs and slots are connected run the following command:
$ wifi-ap.config set disabled=false
This will mark the access point as being enabled.
Now you have an access point with the SSID Ubuntu spawned up and any WiFi device can connect to it.