ARMLIB is a collection of libraries for ARM microcontrollers, in particular, chips with ARM7TDMI, Cortex-M0 or Cortex-M3 cores. In addition, there are a handful of header files and utilities that are geared towards the LPC series chips from NXP and STM32 chips from ST Microelectronics.
The collection compiles with (cross-) gcc and assumes that you run a unix-ish environment. If you are using one of the commercial development environments on Windows, it is completely useless for you.
In addition, this is open source, free software and the usual disclaimers apply: it comes "AS IS", there is no guarantee or warranty of any kind, you use it at your own risk, the authors and the copyright holders under no circumstances can be held liable for any outcome of your using the library and so on and so forth. It is all spelled out in the actual lincence.
If you are still interested, please keep reading.