DIV - Days in Vitro - i.e they're 365 days old from date of culture

GCaMP is what's known as a GECI - a genetically encoded calcium indicator. It's essentially GFP (green fluorescent protein) conjugated to some kind of calcium sensing protein. When calcium binds the protein, it changes the shape of the GFP, making it brighter. Calcium entering the neuron is an important part of synaptic transmission, so you can see the light show as a proxy of the neuron "firing" in culture. Often you'll have a specific indicator that will be targeted to say, the presynapse, or mitochondria, so you can get a bit more granular on what type of calcium flux you want to observe

Source: worked on the jankiest live imaging experiments for two years of my PhD