What is this number

Those are called your timings or also called latency.

Timings measure the time the memory chip delays doing something internally. The most known part of those numbers is the first number which is the CAS latency (or CL or “access time”) it tells how many clock cycles the memory module will delay in returning a data requested by the CPU. A memory module with a CL 4 will delay four clock cycles to deliver a requested data, whereas a memory module with a CL 3 will delay three clock cycles to deliver it. While both modules may run at the same clock rate, the second one will be faster, as it will deliver data sooner than the first one. The other number arent as important as the first one but the rule of thumb is the lower the timings the faster the ram is.

The first number
CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and it returning it.

The Second number
tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.

The Third Number
tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the begin of the access the another line of data.

The Fourth number
tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.

Sometimes you will also see a 5th number when looking at Ram
CMD: Command Rate. The time it takes between the memory chip having been activated and when the first command may be sent to the memory. Sometimes this value is not informed. It usually is T1 (1 clock cycle) or T2 (2 clock cycles)

What is a clock cycle? I am just gonna give you the definition from my computers dictionary

Clock Cycle- The clock cycle is the time between two adjacent pulses of the oscillator that sets the tempo of the computer processor. The number of these pulses per second is known as the clock speed, which is generally measured in MHz (megahertz, or millions of pulses per second) and lately even in GHz (gigahertz, or billions of pulses per second). The clock speed is determined by a quartz-crystal circuit, similar to those used in radio communications equipment.
Some processors execute only one instruction per clock cycle. More advanced processors, described as superscalar, can perform more than one instruction per clock cycle. The latter type of processor gets more work done at a given clock speed than the former type. Similarly, a computer with a 32-bit bus will work faster at a given clock speed than a computer with a 16-bit bus. For these reasons, there is no simple, universal relation among clock speed, "bus speed," and millions of instructions per second. Definition from WiKi

I know this sounds complicated but just remember lower timings means faster ram
SAAIELLO - ".....lower timings means faster ram "

That is not always the case. Kingston just announced the world's fastest DDR3 2400 memory. The memory runs at 9 11 9 27 2 timings at 1.65 volts. Kingston indicated the memory was tested on several P55-based systems including the Gigabyte GA-P55A-UD4P motherboard. The memory is not yet available for purchase and there are no technical reviews yet. I'll bet the modules will be expensive.