The lfo node is a [patch-network] node in web synth that can be created via the [graph-editor]. LFO stands for low-frequency oscillator, and that's just what it does. It outputs values in the range of [-1, 1] in various selectable patterns. It is useful for varying params of different [module]s.
You'll usually want to use the LFO node in combination with a [scale-and-shift] node to map its output into the range needed by a destination parameter. This is contrary to the way that modular synthesizers work, where CV is standardized into volts-per-octave or something similar.