Training frequency is how often you train a muscle or muscle group per week. If you train chest on Monday and Thursday, your chest frequency is two. It is not about how often you go to the gym overall, but how often each muscle gets worked.
What the studies say
A meta-analysis by Schoenfeld, Ogborn and Krieger (2016) found that training each muscle twice a week led to more growth than once a week. A larger analysis by Pelland and colleagues, however, found only a possibly negligible effect of frequency on muscle growth when volume was equal.
Taken together: what mainly counts is the number of hard sets per muscle per week. Frequency is first and foremost a tool for spreading those sets sensibly.
Why distribution still matters
Every set should be hard enough. If you pack all the sets for one muscle into a single session, the quality of the last sets suffers. Spread over two or three sessions, each session stays shorter and each set fresher.
Example: 12 sets of chest per week means 6 sets per session over two sessions, or 4 sets per session over three. These numbers are an example.
Which split gives which frequency
Which plan fits depends mostly on your week. Full body on three days trains each muscle three times a week and suits people with little time. Upper/lower on four days gives a frequency of two and is a good middle ground for most. Push, pull, legs on six days also gives two, but spreads more volume and needs a lot of time and good recovery.