1. If you mean what 1.5 degrees is above, it's an anomaly, which is defined as the difference from an average computed over a somewhat arbitrary time window. I think this one is defined as 1850-1900, which is defined as "pre industrial" times. Britain and America might disagree about that classification, but this is how it's done.
2. If you mean what 1.5 degrees is below, the answer is nothing. A 1.5 degree limit is a goal designed to motivate politicians. It's not justified by any particular research. It is discussed in this interview in Der Spiegel with an IPCC climatologist [1]:
SPIEGEL: Why then have you, as one of the creators of the two-degree target, imposed such a magical limit to which all countries must slavishly adhere?
Schellnhuber: Politicians like to have clear targets, and a simple number is easier to handle than a complex temperature range. Besides, it was important to introduce a quantitative orientation in the first place, which the 1992 Framework Convention on Climate Change managed to elegantly wangle its way out of. And let's be honest: Even if we aim for the two-degree target, we'll end up somewhat higher. Whenever there's a speed limit, most drivers tend to go a little faster.
[...]
And of course the world won't end if temperatures go up by 2.01 degrees, let alone end suddenly. From today's scientific perspective, we could possibly live with a warming of two to three degrees
[1] https://www.spiegel.de/international/world/german-climatolog...