Rows of numbers correspond with sequences of coloured squares. In some puzzles two different shades are used (dark and light). Two sequences of the same shade in the same row or column are always separated by something else.
Consider the number 3 on top of the first column. Wherever we put a sequence of 3 consecutive blue squares, the middle square will be occupied. The same holds for the last column, and the top and bottom row. Looking at the middle row and the middle column, we see that there need to be two sequences of length 1. Those are already present, so we can fill the remainder with crosses (with the right mouse button) to indicate they will remain empty.
In the second column we are required to make two sequences of length 2. Because of the empty square in the middle, there is just one way to put them there. Note that we could have drawn this conclusion right at the start as well, since the two rows of length 2 were not allowed to be connected (that is, they should be two rows of 2 squares and not one row of 4 squares). The same reasoning holds for three other occurences of two 2's and that completes the puzzle.
This puzzle type also goes by the name of "Paint by numbers".
Puzzles in this genre