A plain weave is one in which the warp (vertical thread) and weft (horizontal thread) are simply threaded over and under each other.
Examples of plain weave fabrics
Birdseye - Birdseye fabric is composed of a small repeating raised diamond pattern. It adds a touch of texture but looks like a solid—except when viewed up close.
Nailhead - Nailhead fabric is very similar to the Birdseye but with a more 'square' pattern.
Seersucker - Seersucker fabric is weaved in such a way that the threads pucker, meaning that only part of the cloth is touching your skin. This makes it very breathable, which is why it's commonly worn during the summer months.