When a sequence is started, it is always associated with a sequencer on which it is started. The m_sequencer handle contains the reference to the sequencer on which sequence is running. Using this handle, the sequence can access any information and other resource handles in the UVM component hierarchy.
P_sequencer and how it is different from m_sequencer:
A UVM sequence is an object with limited life time unlike a sequencer or a driver or a monitor which are UVM components and are present throughout simulation time. So if you need to access any members or handles from the testbench hierarchy (component hierarchy), the sequence would need a handle to the sequencer on which it is running. m_sequencer is a handle of type uvm_sequencer_base which is available by-default in a uvm_sequence. However, to access the real sequencer on which sequence is running, we need to typecast the m_sequencer to the real sequencers, which is generally called p_sequencer (though you could really use any name and not just p_sequencer).
Here is a simple example where a sequence wants to access a handle to a clock monitor component which is available as a handle in the sequencer.
class test_sequence_c extends uvm_sequence;
if(!$cast(p_sequencer, m_sequencer)) begin
`uvm_fatal(“Sequencer Type Mismatch:”, ” Wrong Sequencer”);
my_clock_monitor = p_sequencer.clk_monitor;
class test_Sequencer_c extends uvm_sequencer;