Logjam is a new security vulnerability that has been in the news recently. Logjam affects customers using SSL / TLS with weak cryptographic ciphers available on web servers. If these weak ciphers are not disabled, a man-in-the-middle attack can be used to downgrade a strong cipher suggested by the browser to a vulnerable cipher.
This vulnerability affects all browsers except Internet Explorer 11. I have seen reports that Chrome 45 has been released on the developer release channel that also protects against Logjam. These "fixed" browsers protect against Logjam by denying access to the website if a weak cipher is suggested by the server. The default set of ciphers used by Tomcat include weak ciphers.
For full details on the vulnerability and the configuration necessary to protect your server, see this paper on Mitigating-LogJam-in-SBM.