What does the symbol d mean

[God-Ray]

Hello, very glad to read your wonderful project mcl_pi_gazebo. And Im confused that  what does the symbol d and PF_l  PF_t   mean in MCL_PI.py? 

[ortslil64]

Mcl = Monte Carlo localization PI = particles Intersection PF = particle filter

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[God-Ray]

Thank you for your reply!
While I mean what do these symbols stand for in the MCL_PI.py codes.

if recive_particles:
print "recived particles!"
fusion.PI_t = particles
d = np.linalg.norm(np.mean(particles[:,0:2],axis=0) - fusion.mu_t_hat)
print "d= ", d
if d < 0.7:
PF_l.weights = fusion.update_weight()
PF_l.resampling()
recive_particles = 0

When I run the mcl_pi_move_base.launch, I see the d values "d=~~~~“ in the terminal.
There are few annotations in the codes, it is hard for me to understand the codes.
Hope for your reply, thank you very much!!!