There are two numbers available on the periodic table of elements that are really important to know in terms of navigating the table. The first one is the period number, the one you asked about. If you get a copy of the periodic table, look to the extreme left, where hydrogen sits in the uppermost left spot. If you look to the left of hydrogen and all the other elements there in the first column, you will see the rows of elements are numbered straight down the left side, much the same way you would number your paper if you were taking a test. These numbers represent the different periods, and there are about 7 of them.
The other number is the family, or group number. This one is the list of numbers starting with hydrogen, then proceeding across the top, numbers 1 through 18. Everything in that respective group, top to bottom, has similar physical and chemical properties. Group 1 is alkaline metals, while all the way across to the other side is group 18, the noble gases.