To become shorter or smaller (of distance, gap, time, etc.) — an intransitive verb. The transitive counterpart is 縮める.
両国間の距離が技術の進歩で縮まった。
The distance between the two countries has been reduced thanks to technological progress.
点差が縮まって、試合は最後までわからなかった。
The point gap narrowed, and the match was undecided until the end.
締め切りまでの時間が縮まってきたので急がなければならない。
The time until the deadline has shortened, so we need to hurry.