Clocked is an electronics term and here it applies to the LFSR's implementations;
Assume that the LFSRs are implemented with D-FLip-Flop that can store values in bits.
At each clock, the waiting input (D) passes the output (Q) and in LFSRs that go to the next cell. This is the shift operation.
The clock of the system simultaneously acts on D-fLip-flops ...