From The Aquarium Wiki
This is short for the Estimated Index and was invented by Tom Barr.
It is a simple method of adding regular amounts of fertiliser to planted aquariums virtually every day followed by a large water change weekly so that the aquarist knows that the plants have sufficient nutrients to grow healthily and so prevent unwanted algae growth.
To quote the site:
The Estimative Index (EI) is a straightforward method for providing nutrients for a planted tank. The idea behind EI is simply introducing an excess amount of nutrients within an aquarium, throughout the week. This excess of nutrients floods the water column and feeds the plants. This is an estimative method; measuring specific nutrient uptake rates is not necessary and no test kits are involved. EI provides a surplus of nutrients that helps to prevents plant deficiencies, and allows plant growth unhindered. Most algae related issues are due to plant deficiencies rather than excess nutrient levels(Ammonium/NH4 + is the exception).