When we think about frequencies it can be helpful to think about water ripples and waves. I like to think about being at a lake, watching the water hit the shore and seeing the occasional motor boat pass by. I'll often throw or skip a rock into the lake and watch how the water ripples out from where the rock hits. Ripples are basically many tiny waves. If we watch ripples at the shore we might see many little waves hitting over a short period of time. Larger waves, like ones created by a passing boat, may only hit the shore a few times a minute. The little ripple waves are more frequent, so we say they have a higher frequency. The larger waves from the boat are less frequent, so we say they have a lower frequency. In the picture above we see one large frothy wave among a bunch of smaller waves, the large one is less frequent (lower frequency).
The sounds we hear are waves of air molecules hitting our ear drums. We measure wave frequency in hertz, which stands for cycles per second (or in our water analogy, waves-hitting-the-shore per second). If the waves come at too high or too low of a frequency we can't hear them (Humans can only hear frequencies between 20 Hz and 20,000 Hz). A great illustration of how exactly we hear sounds, specifically electronically generated ones, has been made by Animagraffs (I'm a little in love with it).
Oscillators generate frequencies. Above you will see the Low Frequency Oscillator (LFO) which can output 5 different wave shapes simultaneously, with a range from low audible tones to frequencies well below what we can hear. This module specializes in the very low, sub-audible frequencies, and is intended for use as a control voltage source. Using its low frequency waves we can vary the pitch of a voltage controlled oscillator, the gain of a voltage controlled amplifier, or the cutoff frequency of a voltage controlled filter.