Reflow profiling/soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint.
developing a reflow profile is about balancing two variables, time and temperature. If its too hot, you damage parts, if its too cold, the solder won't melt. If the belt speed (time) is to rapid your temperatures will ramp to fast, if its too slow, your flux will burn off prematurely.
There is typically limited range of values for belt speed and zone temperature that will produce a good profile. This range of values is the Process window.
TBH I think perhaps the OP deserved it, there is perfectly good explanation of Reflow and Process Window Index on Wikipedia for starters and if you want more all sorts of FREE specialist papers on things like reflowing flex circuit, lead/lead free etc both on this site and others. The question is waaaaaaaaaaaaaaaaaay too broad, most of us in industry probably learned our reflow knowledge on the job with some basic complimentary training, while something of a "dark art" according to some, the reality is more that when it comes down to it there are some clear parameters that mean you will have a pretty good idea what you profile and process will look like for a given board before you even attempt using fancy software, probes or checking the data sheet for every device on your PCB.
Anyway here's some extra "knowledge", there are 3 main types of reflow soldering:
Infra Red: The PCBs go through an oven and are heated by an array of IR lamps, this is potentially quite efficient but has many shortcomings, tall devices cause shadowing, and heavy devices or thick copper planes have a radical effect on how well the process works, this can result in uneven soldering, hot spots, cold spots and it is somewhat sensitive to colour. A white PCB will solder differently to a green one if if they are the same. IR ovens are almost non-existent.
Vapour Phase: This has long being hailed by many as the ultimate reflow process, it is not possible to overheat components as the reflow happens within a vapour that condenses on the PCB, this means the max temp of the board and the parts on it is the boiling point of the fluid in the oven. These can be conveyor based or use some other board handling mechanism depending on just how big they are. However they have never truly taken off and are not compatible with some devices, notably electrolytics. As a rule they are complex and expensive but the small ones are excellent for a lab or prototyping line.
Force Air convection: Again a conveyor based oven that heats up air and blows it onto the PCB. The oven can be varying degrees of complexity and cost. High end models with go to great lengths to provide closed loop control of every aspect, low end models really are as basic as you can imagine them to be
My response could be seen as "trolling", if so I apologize. I don't normally respond sarcastically (to posts), but it seems like posters sometimes want answers handed to them instead of doing a little research online beforehand to gather knowledge. It is out there and, as I have found in doing the research you may gain knowledge on other issues you are facing.
I actually had to look up the definition of troll to determine that I was the one out of line. As I have learned in these situations, best to keep my mouth shut.
Speaking specifically about solder reflow, the process window should be based on the solder paste you are using. The link above is an AIM lead free product. The paste manufacturers provide general reflow profiles, or a process window to start from.
Your PCB size, thickness, complexity, component density may require "tweaking" of that process window for optimum results.