What does it stand for in a practical sense? See a pic attached.

[CiscERsang]

Hello everyone,

I mean f = 2,75....KHZ that is located at the top.
Of course I know that is a frequency. Is it a frequency of what? How many times per second the trigger has been triggered. Or?

[StillTrying]

"How many times per second the trigger has been triggered."

Yes, when the trigger is cleanly triggering once per cycle of the input signal it's the same as the input signal's frequency.

[CiscERsang]

"How many times per second the trigger has been triggered."

Yes, when the trigger is cleanly triggering once per cycle of the input signal it's the same as the input signal's frequency.

and in a practical sense?

[StillTrying]

"and in a practical sense?"

A useful 2 or 3 digit frequency counter ? I use it all the time to check the triggering's working and the frequency is what I expect.

[CiscERsang]

"and in a practical sense?"

A useful 2 or 3 digit frequency counter ? I use it all the time to check the triggering's working and the frequency is what I expect.

 a frequency of what? For example, I see a single pulse on the screen and triggering on its e.g. pos. edge. Also I see at the top f=3KHZ. Does it mean that this pulse appears with a frequency = 3KHZ?

[AVGresponding]

If it's a single pulse, the scope is extrapolating a nominal frequency from the pulse's on time

(probably) 

[StillTrying]

Also I see at the top f=3KHZ. Does it mean that this pulse appears with a frequency = 3KHZ?

Yes.   If you have only one pulse displayed on screen or lots of them it'll still show the frequency of the pulses.

[CiscERsang]

Also I see at the top f=3KHZ. Does it mean that this pulse appears with a frequency = 3KHZ?

Yes.

if it's so, I find it as a quite useful stuff!    Thank you!

[CiscERsang]

Hello again,
here is a frame which is triggered with f=19.2kHz, if trust the picture. Is it plausible value? Does it mean that the frame appears (is being sent) ~19000 per second?
What do you think about?

[Rerouter]

I would use cursors to confirm here, it is more that your trigger level is crossed at that rate each second.

[StillTrying]

"here is a frame which is triggered with f=19.2kHz, if trust the picture. Is it plausible value?"

It's the total number of rising edges per second, so it's the number of bits per second rather than frames.
The Y amplitude is a bit low, because...

The "Trig Level" is actually 2 levels with about 0.2 Div between them, the waveform's rising edge has to cross both of them, which maybe why you have to have the trigger near the top edge of the waveform.