In the Formula for General Relativity, what does the Lambda mean?

If you're talking about Einstein's Field Equations, Lambda is the "cosmological constant." It represents the energy density of the vacuum and changing its value changes the expansion rate of the universe (loosely speaking). Another way of putting it is that Lambda is the manifestation (in the math) of this mysterious "Dark Energy" you keep hearing about.

At the time, Einstein thought it was a "fudge factor" that needed to be added in in order to make the universe static. Now we know that the universe is not static, but the cosmological constant persists, and has a much larger value than was first thought.

EDIT: Imelda is wrong. Capital lambda does not mean wavelength. In the context that you described, (Einstein's field equations in GR), lambda means what I said it meant.