In general, the lines that begin with
W are warnings, those with an initial
E indicate errors, and
N indicates a Note. For example,
W: http://repo.sinew.in/dists/stable/InRelease: Signature by key B6DA722E2E65721AF54B93966F7565879798C2FC uses weak digest algorithm (SHA1)
E: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/source/Sources 404 Not Found [IP: 184.108.40.206 80]
For these two, there's nothing to be done at your end for the "
weak digest algorithm" - it causes no active harm. However, the "
404" error indicates a failure that should be fixed, as it describes a lack of connection/misconfiguration.
In general, in the Linux/Unix world, silence implies success. If the program issues a message, you should least consider it, and try to figure out what it means.