Some please explain to me exactly what facing is or how to do an understitch(as a sewing term)?

I am trying to do an understitch and I have figured out how to do it except that every time I look it up it says “Clip and trim the seam allowances as necessary and press them toward the facing.” I can’t quite figure out what the facing is that I am supposed to ‘press them towards’. PLEASE HELP! my daughter’s school play is only a couple days away and I need to finish this ASAP! THANKS!

The facing is piece made out of the material that you are sewing it usually lines the waist or neck to make the edge stiff and to give definition. The facing must take the shape of the area that it will be attached to, for example, if it is attaching to the waist it must take the same shape of the waistine. When attaching to the waist it is sewn right side to right side, or wrong side to wrong side of material. The understitch is to keep the facing in place, opening the facing upward and stitching close to the seam and through the seams to keep it in place so facing does not show on the right side when wearing skirt. In your instructions, it says to press toward facing, which means that you should open up the facing when it sewn on correctly and and iron from the tail for the skirt to an upward motion to the facing before folding the facing inside. The ironing upward will keep the facing flat when it is folded inside later. Look up understitch on google and in simplicity.com and you will get more info -bye

How to Understitch a Seam