The Squid developers make periodic releases of the source code. Each release has a version number, such as 2.5.STABLE4. The third component starts either with STABLE or DEVEL (short for development).
As you can probably guess, the DEVEL releases tend to have newer, experimental features. They are also more likely to have bugs. Inexperienced users should not run DEVEL releases. If you choose to try a DEVEL release, and you encounter problems, please report them to the Squid maintainers.
After spending some time in the development state, the version number changes to STABLE. These releases are suitable for all users. Of course, even the stable releases may have some bugs. The higher-numbered stable versions (e.g., STABLE3, STABLE4) are likely to have fewer bugs. If you are really concerned about stability, you may want to wait for one of these later releases.