In a neutralization reaction, you have the addition of an acid with a base. Acids produce a hydrogen ion (H+), while bases produce a hydroxide ion (OH-). When you add an acid to a base, you get the production of a salt plus water. The following is a good example, where you add hydrochloric acid to sodium hydroxide:
HCl + NaOH ---> NaCl + H2O
When you talk about the concentration of the acid or the base, you are talking about the available number of hydrogen ions or hydroxide ions that exist to bond together. There is some heat released with each union of an H+ to a OH- to produce an H2O. The more unions you produce by increasing the concentration, the more heat is produced. So higher concentration has a direct effect on the amount of heat produced. If you add a strong acid in high concentration to a strong base in high concentration, expect to have a significant amount of heat generated as a result ot the reaction.