Think of the RAM as a spreadsheet, with rows and columns. And think of the timings, as the time it takes to access those rows and columns.
CAS - Column Address Select
RAS - Row Address Select
Each number represents a different function:
4.....CL - CAS Latency
4.....tRCD - The time between RAS and CAS signals
4.....tRP - The time between switching a row
12...tRAS - The time between activating and deactivating a row.
CL or Cas Latency is probably the most important part for RAM timings. It is time between it's signal and the data available for processing. So the lower the CAS, the faster it can get and receive data.
tRCD is the time between, a row being selected/activated, and the column in the row being activated, by the CAS signal. In other words, the time between RAS to CAS being accessed.
tRP is the time between accessing different rows. A row has to be activated, in order to access the information. And before the RAM can access another row, the row that was just accessed, has to be deactivated. So in order words, the time between accessing one row to another.
tRAS is the time between the memory getting the signal, to access a row.
I tried my best to explain it, so please correct me if I'm wrong.