Signal is a free, easy to use app, for Android or iPhones, that replaces the default texting app. When sending to other users with Signal installed, it'll perform end to end encryption, meaning the cell carrier and anyone else can not read the message as it passes through the system. If sending to people without the app, it just acts like a normal insecure text.
Signal also supports encrypted voice calls.
Encryption of calls and of texts requires data or wifi connectivity, as communication is routed through the internet rather than through normal voice and text channels.
Of course, no software can replace security culture of a firewall between the aboveground and any hypothetical belowground. But it helps to normalize privacy as a default and makes it harder for corporate or government security forces to monitor everything people do. Documents released by Edward Snowden suggest that encryption methods such as those used by Signal (and by Enigmail for email) have thwarted the NSA in monitoring communications.
We posted in the past about TextSecure for encrypting texts, and you may have heard about Redphone for voice calls. Signal is an evolution and combination of those two precursor apps.