Asslaamu alaykum. Shaykh, my questions are the following:
1. In my college, I do not attend the lectures and ask my friend to sign for my attendance. Is there a sin on me because the teacher thinks that I am present while I am not?
2. Can you explain the meaning of 'Rabitat'; it is mentioned in the last verse of Surat Aal-‘Imraan.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

This act constitutes cheating, lying, and forgery, which are forbidden in Islam. This is a matter that is very clear to everyone to start with, so you must repent to Allah from what you have done by stopping to do so immediately, regretting to having done so in the past, and being determined not to do this again in the future. For more benefit, please refer to fatwa 270745.

As for the imperative verb وَرَابِطُوا (Raabitoo), it means to remain stationed at the frontiers to strive against the enemies of the religion, and it was also said that it means: persevere in the obedience of Allah and remain steadfast on it.

The Tafseer (interpretation of the Quran) of Ibn Juzayy reads:

"وَرَابِطُوا means stay at the guarding posts with your horses, ready for jihad. It was also said: it is the relationship of the slave between him and Allah, meaning making a covenant with Allah to do acts of obedience and abandon sins, and the first meaning is stronger. The Prophet said, 'Ribaat (guarding the frontier) for one day in the cause of Allah is better than fasting a month and standing (in prayer) during this month.' As for the Prophet’s description of waiting for one prayer after another: 'That is indeed the Ribaat,' then this is a metaphor that resembles waiting for the next prayer to guarding the frontier in the cause of Allah for its great reward."

Allah knows best.