Confidence and Supply
This is an agreement when one ruling party agrees to support another in confidence votes for a certain amount of time, typically in exchange for progress on specific files.
If the two parties have enough combined votes, this can assure that confidence votes (such as budgets and throne speeches) will pass. This prevents Parliament from dissolving, which if it did dissolve would trigger an election.
A party might agree to support confidence votes because they are then assured more say in how future legislation is written or discussed, thereby advancing their own platforms and the needs of their supporters.
An important thing to note is that this is not a coalition government. The agreeing party does not attain ruling status and can still vote against (or disagree) with the legislature or decisions made by the ruling party.