CVE-2017-3730

When your computer connects to a secure website or service, it does a “handshake” to agree on encryption keys. With this bug, if the other side is malicious and sends deliberately broken key-exchange settings, some OpenSSL versions can crash your connection/client. The practical result is service interruption (a denial of service), not secret data theft.

This matters to you if you run software that uses OpenSSL 1.1.0 versions prior to 1.1.0d for TLS connections.
It’s triggered by a network handshake: a client connects to a malicious server, and the server’s (EC)DHE key-exchange parameters cause the client to crash. There’s no need for a user to click anything, and exploitation is only relevant when the vulnerable client makes a TLS connection to a reachable malicious server.

Treat this as urgent but not an emergency for the average small business: it is a client-crash Denial of Service triggered during normal TLS handshakes from a malicious server.
However, attackers can use this without logins or user interaction, and the underlying issue is in a widely used component (OpenSSL), so patching should be prioritized for any systems on the affected OpenSSL versions.