Whats the stuff called that is applied to the inside of a T-shirt that has a motif/ applique

[Tillyboo]

It's a sort of nylony mesh patch that looks like it's been ironed onto the inside of the garment to cover up the stitch work on the back of something that has been appliqued or has a motif.
You see it on the inside of kids T-shirts if there is a motif or something on the front. I guess it's to make the finish neater on the inside of the garment.
I've made loads of cushions and laundry bags with hand stitching showing on the inside and I want to hide it so it looks even more neater.
It looks a little bit like bondaweb before it's ironed - but it's not bondaweb.
Hope someone can help me, the school fair is on Saturday

I thought that stuff was just normal interfacing (the iron-on type) to stiffen soft material to allow it to take motifs without going all saggy. I could be wrong though.

Yes, I think that's the stuff. I went into a fabric shop today and they looked at me as if I was mad.
I'll Google interfacing.
Thanks very much