One way to understand strace is to write a short C program and use strace to see what system calls it makes.

This test program allocates memory, uses the memory, prints a short message, frees the memory, and exits.

The strace output shows the system functions this program calls:

$ cat test.c  
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 


main () { 
  char *tmp; 
  tmp=malloc(100); 
  strcat(tmp, "testing"); 
  printf("TMP: %s\n", tmp); 
  free(tmp); 
  exit(0); 
} 
$ gcc test.c 
$ strace ./a.out 
execve("./a.out", ["./a.out"], [/* 51 vars */]) = 0 
brk(0)                                  = 0x9fc000 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc85c7f5000 
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory) 
open("/etc/ld.so.cache", O_RDONLY)      = 3 
fstat(3, {st_mode=S_IFREG|0644, st_size=95195, ...}) = 0 
mmap(NULL, 95195, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc85c7dd000 
close(3)                                = 0 
open("/lib64/libc.so.6", O_RDONLY)      = 3 
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\356\1\16;\0\0\0"..., 832) = 832 
fstat(3, {st_mode=S_IFREG|0755, st_size=1928936, ...}) = 0 
mmap(0x3b0e000000, 3750184, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3b0e000000 
mprotect(0x3b0e18a000, 2097152, PROT_NONE) = 0 
mmap(0x3b0e38a000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18a000) = 0x3b0e38a000 
mmap(0x3b0e390000, 14632, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3b0e390000 
close(3)                                = 0 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc85c7dc000 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc85c7db000 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc85c7da000 
arch_prctl(ARCH_SET_FS, 0x7fc85c7db700) = 0 
mprotect(0x3b0e38a000, 16384, PROT_READ) = 0 
mprotect(0x3b0de1f000, 4096, PROT_READ) = 0 
munmap(0x7fc85c7dd000, 95195)           = 0 
brk(0)                                  = 0x9fc000 
brk(0xa1d000)                           = 0xa1d000 
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 11), ...}) = 0 
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc85c7f4000 
write(1, "TMP: testing\n", 13)          = 13 
exit_group(0)                           = ? 
+++ exited with 0 +++