LFO stands for low frequency oscillator. Oscillators are one of the most important basic building blocks of synthesizers, because most of the time they are generating the sounds that you hear. Oscillators repeatedly fluctuate back and forth between positive and negative voltages, meaning that they are bipolar signals. These fluctuations are called cycles, and the size of fluctuation is referred to as amplitude. The rate or speed of these cycles is called the frequency, which is measured in Hertz, or cycles per second. If the frequency is fast enough, above 20 Hz, it becomes audible to the human ear. Low frequency oscillators generally inhabit the subsonic range below what humans are able to hear, but most can be sped up enough to be audible.
LFOs are commonly used as sources of modulation in a Eurorack synthesizer to make a parameter rise and fall. For example you can use it to control the pitch of an oscillator, the cutoff point of a filter, or the amplitude of a sound.
This brings up an important fact that is a big part of what makes modular synths so exciting. Audio can be used as a control source and some CV can be audible. A big part of what makes a Eurorack system fun is experimenting with connecting things and discovering what happens. Just be sure you’re connecting outputs to inputs and inputs to outputs!