Hailers are short range two-way communicators and serve as the primary method of communication between characters in place of cell phones (which cannot be used).
A hailer's communication function is closer to that of a walkie-talkie than a cellular phone, but many hailers incorporate additional functions.
They can be tuned to specific frequencies, allowing for private chat, but they can also be tuned to public bands that allow hailers to operate as receivers for radio stations such as the one that broadcasts out of the observation deck of the St. Louis Gateway Arch. Some models go even further, acting as a digital recording device and even a low-end scanner. Hailers were initially developed as field radios for the Earth Military Coalition. Their durability has made them indispensable in the new frontier. 
They make a chirping sound when making and receiving communication, leading characters to say things like "chirp me a warning" or "I'll chirp you if I find anything" in place of phrases like "I'll call you."
Notable Usage in the Series
Nolan and Irisa use it as a barometer (device used to detect poisonous gases in the air, signs of radioactivity, etc.) while searching the Arkfall wreckage for materials to salvage.
After someone tries to blackmail Amanda with Kenya's life, Nolan and Irisa tie their hailers to Amanda's so that when the blackmailer calls her, they will get pinged. They planned to triangulate his location based on the hailer's limited range.