Signal encrypts “end to end”. What that means is when your messages are in transit between sender and receiver, it is encrypted and is supposed to be protected against mass surveillance (listeners along the way). Signal is like the envelope with your letter inside, rather than the postcard equivalent of most messaging systems today: anyone who handles your postcard can read your message.
This is good, of course, to protect the privacy during the delivery process. It does nothing for you after you open your envelope and save it to your phone. It’s like leaving your letter on your desk at home, anyone who has access to your phone can read it.
There is an article on why journalists should not use Signal. Notice that mass surveillance is very different from targeted surveillance. If you are the subject of targeted surveillance, you have to use different methods to protect the privacy of your communications.
If you have a particular need, contact me and let’s talk.