What is the meaning of audible and silent prayer, and what are the the limits for a person praying alone to raise their voice in them?

Loud prayer is that in which the worshiper recites aloud, whether as an Imam or individually. It includes Fajr prayer, Jumu'ah prayer, and the first two rak'ahs of Maghrib and Isha prayers. As for silent prayer, it includes Dhuhr and Asr prayers, the third rak'ah of Maghrib, and the last two of Isha. This is by consensus among Muslims. The limit for "loud" is to be heard by the person next to one, but there is no upper limit for it.