What does the notation tt mean to geneticists?

"Tt" would mean that x organism was heterozygous for a dominant allele ("T") on x locus.

A child could get either a trait from their mother or father. Lets say T meant the trait for tall and t meant the trait for short. TT and Tt would mean the child will be tall and tt will mean the child is short. A capital letter is dominant and a lower case letter is recessive. If there is one dominant letter, then the will be tall (if we are talking about the height trait). I hope this makes sense