Cross your t's and dot your i's

I looked it up in my dictionary and ran into a explanation that it means " to deal with all the details when you are finishing something"

I don't get what the "t'd" and "i's" stand for.
and need to know what the "finishing something" mean
does it mean end up something without skipping any trivial things?

Help me here, guys

The t's and i's are the letter "t" and the letter "i." A piece of handwriting isn't complete until every "t" is crossed and every "i" has a dot. "Finishing something" means doing the whole job, whatever it is - writing a report, washing the dishes, painting a house, anything. The saying refers to taking care of all the details of what you're doing, as opposed to leaving things out and doing an incomplete job.