When someone says that a particular piece of music has a nice rhythm, they are referring to the pattern of durations of pitch and silences. Pitch as to the notes played, and silence as the rests telling the performer not to do anything.
Each note value has a corresponding rest value of the same name as well as duration:

In a simple meter of 4/4, the quarter/crotchet is the basic beat unit. When you combine two together, you get a half/minim duration value of two beat units. That can be combined even further into whole/semibreve value, and EVEN LARGER into a breve. Understand that only the whole/semibreve is the largest unit that can fit within the meter of a 4/4 time signature measure.
Similarly, the durations can be divided into smaller units. Dividing a quarter/crotchet in half gets an eighth/quaver. Further in half gets a sixteenth/semiquaver. And so on, and so on…
Notice how as the duration values of the notes progressively get smaller, the more flags and beams (wavy lines and connecting horizontal bars, respectively) there are attached to the stem (vertical line) of the note.
Finally, take a look at the measure below:

See how that there is a dot next to the note head? The dot tells that the duration is 1.5x times that of the original. A double-dot tells that it 1.75x times in durational length.
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