Strace is a system call tracer, i.e. a debugging tool which
prints out a trace of all the system calls made by a another
process or program.
The program to be traced need not be recompiled for this, so
you can use it on binaries for which you don't have source.

System calls and signals are events that happen at the user
or kernel interface.
A close examination of this boundary is very useful for bug
isolation, sanity checking and attempting to capture race
conditions.