Lossy compression main purpose is to reduce the original image size as much as possible by eliminating data from the original image permanently. In other words, some pixels are removed from the original image permanently. This process is irreversible results in a significant image size reduction and a degraded image quality.
Lossless compression main purpose is to reduce image size while retaining the image quality by trying to compress the image without the need to remove any detail from your original image. This process results in a larger image size compared to lossy image compression.