Sites which have those symbols are encrypting your connections to their servers. This means any and all information transferred between you and the service cannot be monitored by third parties. By policy, the BuyFind engine only filters in websites that secure their connections using this methodology.
For a technical explanation, see below.
Hyper Text Transfer Protocol Secure (HTTPS) – Source
Hyper Text Transfer Protocol Secure (HTTPS) is the secure version of HTTP, the protocol over which data is sent between your browser and the website that you are connected to. The ‘S’ at the end of HTTPS stands for ‘Secure’. It means all communications between your browser and the website are encrypted. The HTTP or HTTPS client, such as a Web browser, establishes a connection to a server on a standard port. However, HTTPS offers an extra layer of security because it uses SSL to move data.
The principal motivation for HTTPS is authentication of the accessed website and protection of the privacy and integrity of the exchanged data. It protects against man-in-the-middle attacks