The word “scenarios” refers to a number of different things. It could refer to emission scenarios; it could refer to resulting estimations of how the climate will change in response to those scenarios. Emission scenarios are developed to cover a range of possibilities. They start with current atmospheric concentration and rates of emission and build from there, proposing a range of possibilities.
The new IPCC report will use a new set of emission scenarios rather than the so-called SRES emission scenarios, where concentration pathways will be specified starting with today's concentrations, either increasing or eventually decreasing concentrations, and stabilizing at some level. These span a large range of possible atmospheric concentrations of carbon dioxide, eventually stabilizing at very low levels or at levels that are much above today's levels.
These are specified to climate models. Climate models are then run to translate these changes in atmospheric composition into changes in climate. Very often when you hear of a projection of a 4°C rise in global mean temperature by 2100 that's an estimated response to a change in greenhouse gas concentrations built up over a period of time. This kind of information is then used to conduct impacts and adaptation research to understand what the impacts will be.
I hope that goes part of the way toward answering your question.