There's a manpage
Signal Value Action Comment
SIGTRAP 5 Core Trace/breakpoint trap
This signal (as the manpage suggests) is primarily used by debugging tools like
strace, respectively the
ptrace(2) system call. If one process is tracing another process's activity through
SIGTRAP will be heavily used to »interrupt« the child, even though the child in most cases won't effectively know this happened (a process monitored through
ptrace(2) does not necessarily notice which signal had been sent to it, because of the tracer being able to intercept and filter delivered signals and
SIGTRAP primarily serves to inform the tracer that something noticable happened).
Looking at the
ptrace(2) manpage is very enlightening in this case.