Merge is the mechanic in its plainest form: two identical items combine into the next one up, and the goal is to keep climbing that chain without running out of space.
Board space is the real resource. Every merge frees a square, and every item you place without merging costs one — which is the entire tension of the genre.
Two identical items combine into the next tier up, which frees a board square and moves you along the chain.
Merge low-tier items as soon as they appear rather than letting them accumulate, and always keep one square free.
Yes — drag to merge. Nothing to install.