Some cells start out with (usually encircled) numbers from 1 to 8 inclusive; these are the "islands". The rest of the cells are empty.

Hashiwokakero is played on rectangular grid with no standard size, although the grid itself is not usually drawn. The goal is to connect all of the islands by drawing a series of bridges between the islands. The bridges must follow certain criteria:

They must begin and end at distinct islands, travelling a straight line in between.
They must not cross any other bridges or islands.
They may only run orthogonally (i.e. they may not run diagonally).
At most two bridges connect a pair of islands.
The number of bridges connected to each island must match the number on that island.
The bridges must connect the islands into a single connected group.

Source: Wikipedia (https://pl.wikipedia.org/wiki/Hashiwokakero)