Slates are where a group of like-minded candidates decide to join together as a team to participate in the elections together. They usually have a "slate name" and a slogan too. They'd usually encourage voters to vote for each member of the slate rather than encouraging voters to just vote for themselves as an individual.
Slates can also share budgets and resources (but there will be a deduction in the amount of budget they get to make it fair!).
For a full breakdown of rules on slates, see the candidates' guide.