These files basically inform the tool of the authorities you trust. Certificate authorities (CAs) digitally sign any certificates that they issue using their private key, and to verify this signature the equivalent public key, contained within the CA's certificate, is required. There are default files included, but if you have a specific Certificate Authority you wish to add you must browse to the folder containing its certificate.
It is necessary to have a certificate available that will authenticate resources you wish to use: in particular, MyProxy servers. Globus (provider of various grid software) requires a 'signing policy' in addition to each CA certificate. This should be included along with the certificate in the same folder.
The browse facility allows you to select a folder which contains the certificates you want to use. UK e-Science certificates are provided by default, but you can add others to the folder or choose different folders as well.