It uses the form of:

  WIFI:T:WPA;S:{ssid};P:{password};;

Can generate these on Linux with the qrencode program.

Wikipedia has information on this https://en.wikipedia.org/wiki/QR_code#Joining_a_Wi%E2%80%91F...

Section of the Wikipedia article:

Joining a Wi‑Fi network

By specifying the SSID, encryption type, password/passphrase, and if the SSID is hidden or not, mobile device users can quickly scan and join networks without having to manually enter the data. Note that this technique is valid for specifying only static SSID passwords (i.e. PSK); dynamic user credentials (i.e. Enterprise/802.1x) cannot be encoded in this manner.

The format of the encoded string is:

  WIFI:S:<SSID>;T:<WPA|WEP|>;P:<password>;H:<true|false|>;

Order of fields does not matter. Special characters """ (quotation mark), ";" (semicolon), "," (comma), ":" (colon) and "\" (backslash) should be escaped with a backslash ("\") as in MECARD encoding. For example, if an SSID were "foo;bar\baz", with quotation marks part of the literal SSID name itself, this would be encoded as: WIFI:S:\"foo\;bar\\baz\";;