what happens with this sql code?

I didn't understand what the following code does, I am new to sql, can some one explain in detail?

A = SELECT column1, column2, column3, column4;
B = SELECT column1, SUM(column3) AS Catcol3, SUM(column4) AS Catcol4;

Col1Summed = SELECT A.*, Catcol3, Catcol4 FROM A INNER JOIN B ON A.column1 == B.column1;