What is DTMF?

DTMF: Dual-Tone Multi-Frequency signalling

The keypad system that replaced the rotatory dial becoming the standard for hard lines and mobile devices.

DTMF is the basis for your telephone system whereby when pressing a digit on the keypad you generate two tones simultaneously.

This system usually uses a set of 8 audio frequencies transmitted in pairs to represent 16 signals. There are 10 digits, the letters A-D along with the # and * symbols.

When you press digit 1 on the keypad you generate the row: 697 Hz and the column: 1209 Hz together at the same time.

Mark and Space: Mark refers to the duration of the pressed DTMF tone and Space refers to the silence between DTMF tones.
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People can only dial numbers at a certain speed, this is why you can get programmable autodialers which can store and forward the numbers you want to dial very quickly. Examples of such can be seen during political events in the USA and Canada. Robocalls, which are computerized autodialers, are mainly used by political parties during their campaigns to deliver pre-recorded messages to their constituents.
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DTMF tones aren't solely related to generating phone calls to reach another person, they can be transmitted over radio that can control other systems you may own like flashlights and cameras!