When a jacket is constructed, pieces of fabrics are usually stitched together. The stitching results in punctures in the fabric which can allow water to seep in, causing you to get wet inside the jacket. These seams can be sealed via the process of seam sealing.
Seam sealing can be done through the use of a special tape that runs across the seams on the inside of the jacket, or a jacket can be constructed using the process of heat sealing - which may or may not involve stitching fabric together, but always involves the fabrics being “welded” together (or onto another material such as plastic) with heat.
Additional information on seam sealing can be found at: https://triplefatgoose.com/blogs/down-time/how-waterproof-tape-works