The SleepLink is a kind of FunctionLink that, when executed, will first execute its outgoing set, which should result in a NumberNode, and then use the value of the NumberNode to sleep the current thread for that many seconds. The result of execution is a NumberNode holding how many seconds remain for the sleep (currently zero, unless the thread is interrupted by some outside mechanism).
(use-modules (opencog) (opencog exec)) (cog-execute! (SleepLink (PlusLink (NumberNode 1) (NumberNode 2))))
The above will sleep the current thread for three seconds.