weather

Got it.

LIFR - Low Instrument Flight Rules

MVFR - Marginal Visual Flight Rules

But what does that mean?

In the US, there are four categories of weather conditions and different flight rules for each one.

Visual Flight Rules (VFR): Ceiling above 3,000 feet AGL and visibility greater than 5 miles
Marginal VFR (MVFR): Ceiling 1,000-3,000 feet AGL and/or visibility 3-5 miles
Instrument Flight Rules (IFR): Ceiling 500-1000 feet AGL and/or visibility 1-3 miles
Low IFR (LIFR): Ceiling less than 500 feet AGL and/or visibility less than 1 mile

The differing flight rules effect what ratings you need to fly in a given condition, what equipment you must have in the airplane, under what type of ATC control you must be, and controller separation minimums on approaches and departures.

Does anyone know where "special VFR" fits in here?? The big airports
do not allow special VFR operations inside their Class B airspace. I guess
the weather is sort of "dicey", and you should really be IFR.

What are the specs on "special VFR"?? 🙄

Paiute,
Hope these help.
http://www.ipilot.com/learn/article.aspx?ArticleID=452

The references Radarman gave you explain the situation but they may be a bit dense for folks not used to reading governmentease.

Basically, Special VFR is a clearance you can request for flying into controlled airspace when conditions are MVFR. The controller is not required to but may allow that clearance, traffic and workload permitting. As you pointed out, it's hard to get a Special VFR clearance in Class B airspace except in very low traffic conditons.

I get it now. 😀