Split Container is a object that allows the user to split a page/form in multiple sections with each section being resizable.
A split container in a WinForms application will allow for a vertical or horizontal split of the form. This split can be 50%/50% or any other percentage. The benefit is that the container can then be resized by dragging the 'bar' inbetween the panels so that one set of controls/information can be better viewed. The split container panels can also be completely collapsed so that a section of the form is hidden.
When resizing each panel gains space at the expense of another panel -> for example increasing panel1 to 75% of the screen space will mean that panel2 has the remaining 25%.
MSDN defines a split container as:
The Windows Forms SplitContainer control can be thought of as a composite; it is two panels separated by a movable bar. When the mouse pointer is over the bar, the pointer changes shape to show that the bar is movable.