LTR stands for Long Term Release. This is also known as a LTSR, short for Long Term Support Release. These releases are supported for a longer time, and are meant to be used in Production Environments, where stability is preferred over new features. In terms of the kernel you are reading about, the LTR cycle is about 3 years. This means if you are a user who needs stability, if you download an LTR kernel, it will be supported upstream for the next 3 Years. The definitive source for Linux Kernels are the Linux Kernel Archives.
