I am doing a report on Dimebon and it says that it "stabilizes glutamate induced calcium signals". What does that mean

Glutamate is a neurotransmitter. That means that it's a chemical "injected" by neurons through the membrane of a cell in order to stimulate that cell into doing something. For example, when your brain tells your heart to beat, neurons adjacent to the muscles in your heart inject neurotransmitters into the heart muscle cells initiating a chemical reaction which causes them to contract. There are lots of different neurotransmitters used throughout the body for all the various stimuli necessary to maintain life. In the case of glutamate, one of the things it can stimulate is the production of calcium ions. Unfortunately, in certain types of brain injury or disease (perhaps including Alzheimer's? I'm no expert in this subject matter), glutamate can be overproduced and accumulate outside of brain cells. This can stimulating the over-production of calcium within the cells, leading to cell damage or death. The process is called excitotoxicity.

So "glutamate-induce calcium signals" probably refers to this unintended excess of calcium ions caused by an excess of glutamate. I'm guessing Dimebon probably tries to impede that process somehow, either by reducing glutamate production or somehow preventing it from stimulating calcium production. Just a guess though.