Land and hold short operations occur when aircraft are instructed to land, but they cannot cross some designated point, often another runway (but it doesn’t have to be another runway). ATC does this to increase the throughput of aircraft at the airport.
When a LAHSO is in effect, you’ll hear about it on the ATIS (“LAHSO in effect”). Then the tower will clear you: “Cleared to land runway __, hold short _____”, and it is the pilot’s responsibility to state whether they can accept that LAHSO clearance or not, and that will be part of the read-back that the pilot gives to ATC (“Cleared to land runway __, hold short _____”).
The pilot can also say “Unable.” If they do, then ATC will follow with other instructions, like a clearance to land full-length at that runway, or tell you to go around and clear you for another runway, or something else. I don’t expect a full arsenal of possibilities in the first implementation of LAHSOs, but it would be a nice touch of realism to start with the possibility of a LAHSO scenario.
This is what a LAHSO looks like on an airport diagram: