In the Advanced panel there are options to add 5’ extensions to primers and to specify a mispriming library.
A 5’ extension can be your own sequence, a restriction enzyme or Gateway site, or a combination of these. For more information see section 13.9 .
A mispriming library is a set of sequences (usually repeats) which the primers should not bind to. Four inbuilt libraries are available for selection, or you can upload a custom library of sequences in fasta format. For more information on the inbuilt libraries, see the Primer3 help page.