You can do a lot with the graphical interface of OpenSesame, but for more complex tasks you will have to use scripting/ programming. This is where the inline_script item comes in.
You can use regular Python code in your inline script. Python is a modern, powerful and high-level programming language. There are numerous libraries available for Python, so there is no really no limit to what you can do using inline_script items. For more information about Python, see http://www.python.org.
The inline_script item contains two scripts. The Prepare phase script is called first, before the sequence that contains the inline_script is actually executed. This allows you to do perform some time consuming tasks in advance, so that the Run phase goes smoothly. The Run phase script is called during the actual running of the experiment. So if you want to present a stimulus display in your inline_script, it is good practice to construct the display in the Prepare phase and show it in the Run phase. Note that this is purely a convention, from which you can deviate as circumstances dictate.
To get started with inline scripts, you may want to take a look at the example experiments, which can be found on the OpenSesame website:
http://www.cogsci.nl/opensesame#documentation.
An article about using Python inline code in OpenSesame can be found here
http://www.cogsci.nl/blog/tutorials/107-using-python-inline-code-in-opensesame
With the inline_script item you can write Python code using the built-in editor. If you want to include scripts from external files, you should use the external_script plug-in instead.