All target selectors start with @. The following character is the selector type. For example, @p means 'closest player'.
Target selectors may have additional, optional arguments. You can specify these in [ ], using the syntax a=0. For example, @p[a=0,b=5,c=-500].
There is a special short syntax for just specifying x, y, z and r arguments; simply list their values separated by a comma, without x=. For example: @p[100,64,-100,5] for 5 range, x=100, y=64 and z=-100. Each of these are optional and skippable by leaving them empty. For example, to just specify y coordinate: @p[,64].
x- X coordinate for search center. Default is senders coordinate, or 0.y- Y coordinate for search center. Default is senders coordinate, or 0.