Commit records script step explicitly causes the data to be immediately saved to the database...
Lets say you have a simple script that does nothing more than set field and then exits.
If the user does nothing after this script executes the data change within the script will not be saved until they take some action that causes a commit.
This is an important concept to understand especially in multi user systems. Its also useful to know how commit happens with any script steps that edit records.. i.e. import, replace field contents, show custom dialog... etc