First of all, let take a look about what's a 'Mainline', 'Stable', 'Longterm' and 'Distribution' kernel. Here is the introduction form kernel.org.
Then, let's see what is a Ubuntu Mainline Build kernel.
What does a specific Ubuntu kernel version number mean?
The official version of an Ubuntu kernel tells you a number of things, including the base upstream version, the current Ubuntu ABI identifier and the kernel flavour. (See How can we determine the version of the running kernel? to find your current version number.)
Given a version like 2.6.35-6.9-generic this can be broken into four parts as below:
<base kernel version>-<ABI number>.<upload number>-<flavour>
The base kernel version represents the mainline version on which the Ubuntu kernel is based. The ABI number represents significant changes in the kernel Application Binary Interface. The upload number is a monotonically increasing counter for each upload of this base version. The flavour indicates which kernel configuration variant this is.