Trading platforms like mtgox and bitstamp display the current bitcoin price on the top. How they calculate that and what effect it has when trading? What labels Last|High|Low and Vol actually mean?
I do not understand what do you mean by the current bitcoin price on the top (I can not see it), but I will clarify on some other numbers.
Last - the price of bitcoin based on the last transaction. So if I bought 1.2 bitcoins with a price of 950$ and this is the latest transaction so far, this number will appear in the last.
High - What was the highest price for the day. Bitcoin price fluctuates and if you assume that during the day the price went from 900 to 920, dropped to 870 went to 980 and right now it is 950, the highest per day was 980. This number you will see in high.
Low - the same idea as with high, but the lowest number. For this example it will be 870.
About Volume, I might be wrong so if there is someone who knows better, please edit my post. Volume this is the total number of bitcoins which are putted on sale.
Therefore the weighted average is calculated in the following way. Assume you have 3 guys putting their prices on bitcoin. First one wants to sell 1B for 900, second 3B for 910, third one 2B for 890. The weighted average will be (1*900 + 3*910 + 2*890)/(1 + 3 + 2) = 901.666