This is what it's all about. Without the log command you don't know what your participant was doing. Every session – one participant running one script – gets a log, and these commands write a value to the log.
To provide the maximum of flexibility for those who need to control everything about the format of their log files, PsyScript does not automatically put anything at all in the log file. Consequently it is usual to use a procedure like the following in your script and to call it early on in your main procedure. You can copy this one straight out of this help page and paste it into your script if you want.
log value
Writes the value to the log, delimited by 'tab' characters.
log Doing focus point procedure.
log $varCounterSoFar
log array value
Writes every element of an array to the log, as if they were each logged singly.
log array $varSortedResponses
logln value
Does the same thing as the log command, except that the next thing logged after this one will appear at the start of the next line. Has the same effect as following a normal log command with log $return.
logln Doing focus point procedure.
logln $varCounterSoFar
None.