PCR (Polymerase Chain Reaction) is a very useful tool in molecular biology to amplify specific DNA fragments. Briefly speaking, to perform PCR, one needs to design two PRIMERS (short DNA sequences), which will act as "hook" for enzyme to start DNA replication. For the whole process to be efficient, primers need to have similar (± 5 °C) melting temperatures Tm and this app allows to calculate them. Simply enter sequences of PRIMERS (adenine A, cytosine C, guanine G, thymine T letters are allowed). You can also include salt concentrations in your reaction mix, because it affects melting temperatures.
As a bonus, the app shows a barplot of frequencies of each app, the primers length and GC content (it also affects Tm).