There are two types of water drainage called weep holes. There are weep holes in the shower drain in the floor beneath the tile, and there are (should be!) weep holes at the bottom of the tile installation on shower walls above a tub or acrylic shower base. They both do the same thing (allow water behind the tile to drain), needed for different installations. I will address both here.
Weep holes in shower drains beneath shower floor tile
In a traditional shower floor (with a pre-slope, rubber liner, top slope then tile) weep holes are required in the shower drain. The top mud deck, or top slope (the deck mud onto which the tile is installed) will remain saturated with water (if the shower is used regularly). This is called a water-in, water-out system. Water will get behind your wall tile, and beneath your floor tile. When properly constructed your shower has the waterproof liner installed over a pre-slope, then deck mud is installed over that with tile bonded directly to it.
The water that gets behind your tile will run down your waterproofing and into the floor liner. That water will soak into the deck mud on the top slope. This is normal!
With every use, the water will flush through, with new water getting behind your tile and forcing the old water down, eventually into the drain.
This is water-in, water-out.
When unused, gravity will draw that water down toward the drain. Since this layer is below your tile, so below the top opening of your drain, that water needs a way to enter the drain. That is the purpose of weep holes in your shower drain.
Regardless of your brand of clamping drain, the weep holes will be integrated into the upper flange.
A clamping drain is called that because the rubber liner…wait for it… is clamped between the upper and lower flange, creating the seal.
Tile product manufacturers aren’t known for their originality.🤷♂️
This image shows the hole in the liner, centered over the lower flange (place silicone beneath the liner to help seal it). The upper flange is the black ring sitting next to it. The keyhole-shaped holes are placed over the bolts, twisted to lock in place, and tightened down to clamp the liner between the two flanges and seal it. If you look at that ring the four little channels along the edge that run inward toward the four holes are your weep holes. The water runs through those channels and down into the weep holes into the drain.
It is imperative that you do not install plastic behind your walls since this would create two waterproof membranes with your substrate between them. Having two barriers this close together leaves open the chance of trapping moisture between them with no way for it to evaporate. This may lead to mold.You must also tape the backerboard seams with fiberglass mesh drywall tape. 
