What do 2 pairs of three numbers separated by colon mean in SDF files when describing delays of cells?
The reference of the SDF file format contains all information and can be found here. Another good starting source for reading SDF files is this blog entry.

The SDF snippet you posted defines a cell with the name "AND2" and the path to the cell INSTANCE "top/b/d" and is giving DELAY information for it. ABSOLUTE means it is replacing previous delay information. The two IOPATH entries define the delay between the input ports (a/b) and the output y. This information can be found with more detail in the blog entry mentioned above.

To fully understand the IOPATH entries you need to look at the SDF reference. SDF allows to define different delay values dependent on the transition (e.g. 0->1 or 1->0). Therefore you can specify one, two, three, six or twelve delay values. More information can be found in the SDF reference on page 3-16 and 3-17. In this case there are given two delay values.

Each of the delay values can contain one, two or three values. If one value is given it defines the delay from the input to the output (to get the duration you have to consider the TIMESPEC in the SDF header). For two values the first defines the delay and the second the pulse rejection limit and the X-limit. If three values are given the first specifies the delay, the second the pulse rejection limit and the third the X-limit.

The pulse rejection limit defines the maximal width of pulses that do not change the output.

The X-limit defines the maximal width of pulses that leave the output in an unknown state (X). All pulses with a width greater than the pulse rejection limit and less or equal than the X-limit will change the output to unknown.

This information can be found in the blog entry and the SDF reference on page 3-18. The reference lacks a description of the pulse rejection limit and X-limit but the blog provides a description.