You might find that tapestry weavers use different terms to explain how the weft yarn is woven between the warp ends. The warp ends are the string on the loom.

Since the yarn goes over and under each warp end, my very technical term is the over-and-unders. It's probably the kindergarten teacher in me, but it makes the most sense in my brain.

Sometimes the over-and-unders can get mixed up when you're weaving shapes. 

Here's what I'm talking about.

The blue dots indicate where the yarn went under, and the pink where the yarn went over.

When I weave from the yellow shape to the green, the over-and-unders don't match up, and if I kept weaving the shapes, the warp ends would show. 

There are many ways to approach the problem, and sometimes they get confusing. As you become an experienced weaver, you'll learn different ways to solve it.

But I like to teach beginners an easy way to weave, and not worry about such encounters.

The answer is soumak - the great equalizer!

This tutorial explains it all.