What is the meaning of Arabic letters in the begining of some suras?

For example, Al-Baqara begins like this: > **الٓمٓ** ﴿١﴾ ذَٰلِكَ ٱلْكِتَـٰبُ لَا رَيْبَ ۛ فِيهِ ۛ هُدًى > لِّلْمُتَّقِينَ And also, Ya-Sin begins like this: > **يسٓ** ﴿١﴾ وَٱلْقُرْءَانِ ٱلْحَكِيمِ What are different opinions about these letters among scholars? What are they called?