wait instruction is a single-qubit instruction that may be used to explicitly tell the scheduler to idle the qubit it operates on for the given number of cycles. For example, if you would write
prep_z q wait q, 100 measure q
the measurement must start at least 100 cycles after the prep_z completes. Just like any single-qubit instruction, wait q[0,1] follows the single-gate-multiple-qubit rule, expanding to wait q; wait q. Thus, the wait blocks are independent (different from the barrier instruction).