What does it mean if there are two different alleles for a trait ?

The trait is heterozygous.

Explanation:

Heterozygous: "Having two different alleles of a particular gene or genes."

A heterozygous genotype looks like: Rr. The capital R is a dominant allele, and the lowercase r is a recessive allele. These two alleles are not the same (not two dominant alleles or two recessive alleles) meaning that the trait is a heterozygous trait.

The opposite of a heterozygous trait is a homozygous trait, in which both alleles are the same (both dominant or both recessive) and can look like RR or rr (genotype-wise).

you are certainly talking about this picture:

a: is the label of the white square, it is showing the  replication fork, formed by helicase (d) ==> so it is the site of dna separation and replication 

b: is a new dna brand that is following directly the replication fork, so it is continuous ==> built continuously from 5' to 3

c: dna polymerase cannot replicate dna on its own, it needs the of their proteins, and one of them plays the role of a clamp which fixes  the dna with the dna template and increases  the processivity of the enzyme. ==>   stops polymerase from falling off the  template.

d: is the enzyme responsible for forming the replication fork (helicase) ==> unwinds and separates dna into two strands

e: is the delayed strand, it is built  in the opposite sense of the helicase, so dna will be built in fragments ==> built  fragments from 5' to 3 joins

and finally f: is the enzyme that builds  the new dna strand (dna polymerase) ==>   adds nucleotides to make a new dna helix.

credit: gradman555p98x9p

read more on -