What are high frequencies and low frequencies in a signal?
The high frequencies contribute to the fast varying parts of the signal(the sharp transitions),
while the low frequencies contribute to the slow variations of the signal in the time domain.

You might want to take a look over here:
http://cns-alumni.bu.edu/~slehar/fourier/fourier.html

Also, if you have the time you might want to take a look at this online course:
http://academicearth.org/courses/the-fourier-transform-and-its-applications
You won't regret it.