For you to properly care for your art, you must understand its basic construction.
The traditional construction of a painting on canvas is to stretch a canvas on expandable bars. These stretcher bars are the foundation for the painting.
Canvas, like all woven materials, has a warp and weft: the difference in the direction of the weave. Traditionally a canvas was stretched and then sized with one or more coatings of hot water soluble glue. This sealed canvas shrunk the canvas making it tighter on the stretcher bars then one or more coats of a gesso ground was applied.
This ground coating was also used on panels; it created a smooth surface eliminating the wood texture or the canvas weave. Water may also cause a canvas to shrink; wood panels may warp or crack.