Your retainers can be made to keep the teeth that you have in their proper position. But you build the retainer with acrylic pads in the gaps that you have, that occlude against the upper or lower teeth. They donot let the opposite teeth from the gap move into the gap and they donot let the teeth on the either side move toward one another until you can afford to put bridges or implants in these gaps.
If you donot retain it properly, the teeth on either end will move into the gap and the tooth above or below will move into the gap also.
If you are thinking of getting the suck down retainers, they are cheap and they don't do a good job at all.
Retainers acts like a temporary partial.