What's the accepted, applied, and irreversible transactions mean in mongo db plugin?

Question

In mongodb plugin, there's interface:

void accepted_block( const chain::block_state_ptr& );

void applied_irreversible_block(const chain::block_state_ptr&);

void accepted_transaction(const chain::transaction_metadata_ptr&);

void applied_transaction(const chain::transaction_trace_ptr&);

What does each state mean?

And how to know if an action trance is irreversible?