Energy Density refers to the amount of energy contained in a unit of something. It’s typically used in the context of fuels which can be moved (oil, coal, ethanol, wood pellets etc.), with the units either volume or weight. So energy per kg, energy per litre and so on.
High energy density means you don’t need much fuel (by volume/weight) for it to contain a lot of energy, which is extremely important economically and for scalability.
In the other direction, for example replacing coal in a power plant with much less energy dense wood pellets means that, to generate the same amount of electricity, you’ll need to transport a much larger quantity of wood pellets to the power plant than you did coal. That might not be too problematic if the wood source is available nearby, but what about scaling to very large-scale coal replacement? Where that means transporting over long distances, lower energy density means more boats/trains/trucks are needed to move the same amount of energy (with associated costs, fuel usage, emissions and so on).
Resources like solar and wind don’t involved energy stored in “fuels” that can be expressed in terms of three-dimensional volume or weight. However it is common to describe them in terms of area “density” of energy (e.g. kWh/m2/yr).