This maths worksheet uses the subtraction sign with numbers up to 10. The sign for subtraction could well be new to children of this age. Think of the statement 7 – 4 as a sentence which can be read as:
7 minus 4
7 subtract 4
or 7 take away 4.
A number line can be helpful here. To take 4 from 7 start with the larger number which is 7, and then make four hops back, first to 6, then 5, then 4 then 3.
There is a number line on the worksheet, but it would be better to provide your own.
Many children will use their fingers to count the number of hops backwards they have made, there is nothing wrong with this. Alternatively they can mark the hops backwards on the number line itself.