If the fractions are similar fractions or have similar denominators, you will simply add/subtract the numerators of the fractions and just copy the denominator.
Ex. 1/4+2/4=3/4 because, you add the numerators and just copy the denominator.
If the fractions are dissimilar or have different denominators, use LCD to make the denominators the same, once you make the denominators same, add the numerators and copy the changed denominator just like the similar fractions, you use LCD so you make it similar fractions, LCD is just like LCM.
Ex. 5/8 - 2/6 = 7/24 because 8 and 6 are different, you have to get the LCD,
8 = 2x2x2
6 = 2 x 3
2x2x2x3 = 24
5/8 = 24 divided by 8 times 5 = 15 which means 5/8=15/24
2/6 = 24 divided by 6 times 2 = 8 which means 2/6=8/24
Then 15/24 - 8/24 = 7/24