On March 4, 2017 Salesforce are deactivating the TLS 1.0 encryption protocol. TLS = Transport Layer Security, used to provide secure connections between client applications and servers. TLS 1.0 has a number of known vulnerabilities and is gradually being phased and replaced with TLS 1.1 and/or TLS 1.2. Every connection to Salesforce whether it be from a web browser, email client or a 3rd party application must use TLS 1.1 or 1.2 after 4 March 2017 or will be refused connection.
If you are the administrator of a Salesforce org, you need to start preparing now or your user connections may be impacted.