JPEG is an image format with lossy compression.

This means, every time you save a modified JPEG image, its quality gets slightly reduced because the algorithms used for encoding and compressing the information (namely DCT - discrete cosine transform) do not work pixel-perfect but only store the most important (depending on the quality/compression level) information to reproduce an image that looks basically the same to the human eye, in order to reduce the file size.

To my knowledge, even the highest quality/lowest compression setting available still uses this algorithm, resulting in a minimal lossy compression and loss of information.

You should also note that with relatively high quality levels, the file size quickly shrinks if you reduce them slightly while there is nearly no notable difference. In contrast, a slight change in the range of low quality levels is well visible and only has a low impact on the file size.

If you want to absolutely avoid any information loss, you should work with photos in a lossless format (e.g. RAW or lossless TIFF) and only convert them to a format with lossy compression once in the end if you want smaller file sizes, don't repeatedly save in a lossy format.