question

One of the rationing functions of markets is to ensure that goods are distributed to the people who value them most. Comment on the ability of markets to fulfil that function when the government institutes a price ceiling in a previously unregulated market.

answer: When government institutes a price ceiling, this decreases total surplus and leads to lost transactions and ultimately deadweight loss.

I failed to understand what it means by the deadweight loss, please elaborate on it,

, and what does it mean by total surplus and lost transactions?