I am forgot to take a hydrometer reading when I put the beer into the primary fermenter. Can I take one when I transfer to the secondary and what will it tell me anyway?
You can still take one now, if you just forgot, just be sure to sanitize everything before you take the reading.
You can also gather valuable information if you take a reading before secondary. Since fermentation should be complete before secondary, you can make sure the majority of your sugars have been consumed by checking that your specific gravity is around what you would expect for a final gravity. You can also estimate ABV assuming you have a good estimate for what your starting gravity was.
If you brewed extract, than it should be easy to estimate OG, and FG is really the more important measurement. If you brewed all grain, estimating OG is a little harder if you don't have a good estimate for what your extraction efficiency will be, but is still possible. Find a good online tool to help you (I prefer beer calculus) and you can get a good estimate assuming you still know your recipe.