A file extension is the set of three or four characters at the end of a filename; in this case, .obj. File extensions tell you what type of file it is, and tell Windows what programs can open it. Windows often associates a default program to each file extension, so that when you double-click the file, the program launches automatically.
A threshold (or limen) is the point of intensity at which the participant can just detect the presence of a stimulus (absolute threshold) or the presence of a difference between two stimuli (difference threshold). Stimuli with intensities below the threshold are considered not detectable (hence: sub-liminal). threshold (or limen) is the point of intensity at which the participant can just detect the presence of a stimulus (absolute threshold) or the presence of a difference between two stimuli (difference threshold). Stimuli with intensities below the threshold are considered not detectable (hence: sub-liminal).
Such definitions typically define a forest as an area growing trees above some threshold. These thresholds are typically the number of trees per area (density), the area of ground under the tree canopy (canopy cover) or the section of land that is occupied by the cross-section of tree trunks (basal area).