a/b/g/n/ac refers to variations of the the 802.11 standard for Wi-Fi networking.
a, b and g are older versions and hardly used these days (but are referred to in some newer networking products such as wireless routers for backwards compatibility).
n is the most common Wi-Fi standard (and it incorporates the older b and g standards).
All the models showing with Wi-Fi support in this test support the n standard
ac is the latest and fastest version of Wi-Fi and also a handy feature as it can operate on the less congested 5GHz band.