What is the frequency of a wave? How is it measured? What part of the wave is affected?
Frequency is the number of occurrences per unit of time, e.g. cycles per second.
The wavelength is the distance (length along the horizontal axis) a wave "travels" per unit of time. The period is the length of time it takes for a complete cycle.
For a sinusoid, the wavelength is the period or the distance from crest to crest (or trough to trough or any other point to corresponding point on the graph.) The frequency is the reciprocal of the period. (For the graph of the sine curve, the period is 2pi, and the frequency is 1/(2pi).)
When the frequency changes, the graph undergoes a horizontal dilation -- a dilation factor greater than 1 "squeezes" the graph and increases the frequency (shortening the period and wavelength) while a dilation factor less than 1 decreases the frequency (and lengthens the period and wavelength.)
The way frequency is measured depends on the application -- sound is in cycles per second for example.