IPsec
A protocol suite for encrypting and authenticating IP traffic, most often used for site-to-site VPN tunnels.
IPsec negotiates keys and then encrypts traffic between two endpoints. It is well established, widely interoperable between vendors, and the usual choice for permanently connecting two offices.
Tunnels fail most often over mismatched settings between the two ends, or because one side sits behind a NAT it does not control and cannot accept the inbound negotiation.
Where this comes up
IPsec sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.