At IIT Madras, open electives, also known as free electives, are interdisciplinary courses that you can choose outside of your major. (for example: Computer Science student taking Biological Sciences course, etc.) At IITM, you can customise these electives by upto 40% of your curriculum. You can take these electives from other departments as early as your second year to explore newer disciplines.
That means your curriculum does not restrict you to your core branch. For instance, as an engineering design student, you are free to take finance electives. Usually, for engineering undergraduates, a minimum of three humanities electives (again, preference-based) are made mandatory open electives for them. If you take a series of electives from the same discipline (and in accordance with the approved minor basket), you can convert them into your official minor degree.
Some of the popular open electives include:
Principles of Economics: one of the most popular social science electives in the campus. One of the bases, after which many engineering students have minored in economics.
GN courses: General Category courses focus on life skills and personal and professional development (PPD), with GN6106 (Happiness, Habits and Success) particularly popular, with 600+ students registering each year.
Language courses: Language courses are primarily offered by two departments. The Humanities and Social Sciences Department offers Korean Iⅈ German I, II & III; French Iⅈ and Tamil for Beginners. The Engineering Design Department offers Japanese I&II, and, Chinese I&II.
The ED language electives are also considered as HS electives (so students whose mandatory HS electives have not been met can also pursue these ED language courses)
To learn more about what exactly electives are, check this article and check out more popular electives here.
If you have more specific questions regarding certain electives (their difficulty, accessing past year question paper patterns, etc.), direct them to the AskIITM Community.