Parsing serial port data
It's parsing a bunch of semi-colon separated data, like &quot;12;34;45;&quot;. In each iteration, it converts one value from character representation to its integer value.
strtok_r is the reentrant version of strtok. On Arduino you simply use strtok. Every time you have to parse a string having a list of values separated by some char, you use strtok.
Name
strtok, strtok_r - extract tokens from strings
Synopsis
#include &lt;string.h&gt;

char *strtok(char *str, const char *delim);

char *strtok_r(char *str, const char *delim, char **saveptr);

The strtok() function parses a string into a sequence of tokens (sub-strings). On the first call to strtok() the string to be parsed should be specified in str. In each subsequent call that should parse the same string, str should be NULL.
Each call to strtok() returns a pointer to a null-terminated string containing the next token. This token does not include the delimiting byte. If no more tokens are found, strtok() returns NULL.