In other words, if both parents have a gene for tallness and a gene for shortness (heterozygous), each parent plant has a 50-50 chance of passing either gene on to the offspring independently from the gene the other parent donates.
The other main concept of Mendel's experiments with pea plants defined the nature of sexual reproduction. It is not actually another law, but perhaps this is what your teacher means. This is the concept that we know today as meiosis, or the formation of gametes (eggs and sperm). It means that since each parent can only donate one allele for a given trait, the sex cells are haploid, meaning they contain only half of the parent's pairs of genes for each trait.
Human beings have 46 chromosomes total, which is the diploid number (after fertilization, when egg cell is joined with sperm cell). However, the sperm and egg cells must be formed with only half the number of chromosomes (23) so that the newly fertilized cell has the proper diploid number again (46), and the offspring has one gene from the mother and one from the father for each trait.
I hope this helps a little.