Each 'clock' in a SDRAM module is a physical electrical path or trace on the module. Typically, SDRAM memory modules have 2 clock lines (Clocks) per bank of memory. A two bank module would normally have 4 clocks, while a single bank module would have only 2 clocks.
SDRAM emerged on the market as weak standard that didn't adequately define the number of clock lines per bank. Consequently, some older motherboards actually provide only 2 clock lines to the module interface or socket. In such a system with two bank modules, those two clocks must be divided within the modules so that both banks get their required signals. Putting a normal 4 clock module in a 2 clock system will always result in a malfunction. That's the reason Advantage Memory has designed some modules to be adjustable through the use of soldered jumpers.
The most important thing to remember is that all clock lines on the SDRAM module must receive a clock signal from the motherboard, but each clock signal from the motherboard need not connect to a clock trace on a memory module.