By default, the widths of all columns in tables are "dynamic" – they expand to fit the width of the content they contain and the defined width of the table. After you create a table you can adjust the widths of columns by dragging their borders with the mouse and by setting their widths to absolute or percentage values in the Table Properties dialog.
You can also "lock" the widths of columns. This can be necessary if you are using tables as a layout tool and want to have column widths that do not change dynamically when the users adjusts the size of the help viewer window.
How you define your column widths is crucial for how your tables behave in your published output, in particular in formats like PDF and DOCX where the tables must fit inside a fixed page width.
Table Properties (Reference)
How table sizing works (Reference)