Help what is the purpose of a matte box, and other related items for DSLR's. Am new to filmmaking, and just want to know what to buy and why. I have Canon T3I.
You might be able to find a good deal of information if you search through the site as the "what is a matte box" question has been addressed many, many times. Basically, a modern matte box is used to accomplish two things: keep extraneous light from hitting the lens, where it would create flares and other undesirable contamination effects; allow for the use of external color, diffusion and effects filters in front of the lens.
Originally, when special effects were accomplished in-camera or via optical printers, multiple shots would sometimes be accomplished by matting out sections of the frame in one shot and then taking another pass with the opposite section of the image masked to add another element. Glass shots were another form of matte photography.
Matte boxes are indispensable tools for crafting your image today. Because filters can move on more than one axis and rotate, a matte box allows you to precisely control where the dissolve area of a graduated filter falls within the frame. Graduated ND filters are very important in video work, where cameras historically have less dynamic range.
A matte box with no filter-holding capability is not a matte box, but a sunshade.