What is the physical significance of WGT and SBn in MCNP source specifications?

What is meant by SP1,SI1 and SB here? I actually can't get the physical significance. And What is the physical significance of WGT here? Sorry for my this kind of questions. I am novice in MCNP.

It's sometimes better to look at examples than to read the manual. The full spec often contains a lot of rarely used options. WGT and SBn are closely tied to variance reduction and what is called "The weight window game". If you don't know what the weight window is don't tackle this yet.

That syntax quoted creates a source at a point x,y,z of some kind of particle with energy picked from a distribution D1. The distribution is defined by the parameters in SI1 and SP1 (and SB1 but you can ignore this one for now). The distribution could be a series of bins with the probability of each or a smooth function with a random number as the input.

If a second distribution is needed, D2, that would have SI2 and SP2 etc

Distributions are very powerful and you can also use them to control the source points or direction of the particles. For example if you want the source to be a line or a solid cylinder or sphere feeding the right distributions into the available position variables is usually how that is done.