What happens when you retain a stitch? (photos)

After a BL I started to spit stitches 2 weeks ago. Then the lower part of my R breast became red. My PC said I had retained a stitch. After another day my breast began to fill with blood, and I had to go back to surgery. My PC took out the retained stitches and put a drain in. My question is what is a retained stitch and how can I avoid this happening in future. My PS put more stitches in. I’ve worn my surgical bra day and night. Could that have cause the stitch retention?