It is three one eighth inch marks starting from zero moving from left to right on the ruler. The first six marks
Starting at the mark next to the number 1 (1 inch), go halfway to the end of the ruler and you will find a mark that should be one half of the length of the mark designating 1 inch. This will be the 1/2 inch mark. Again, go halfway to the end of the ruler and you will find a smaller mark that is for 1/4 inch. The next smaller sized marks on either side of the 1/4 inch mark designate 1/8 of an inch. The 1/8 mark between 1/4 inch (2/8) and 1/2 inch (4/8) is the mark for 3/8. Often, the small arks at the end of a ruler or tape measure are difficult to use. In construction trades, many workers will "burn an inch". To measure something that is 3/8" (inch), they will use the distance between 1" and 1-3/8", disregarding the additional inch. This can make it easier to use these tools.
The link below is to a simple text that shows exactly all of the fractions to the 16th
What is the little line just after 2 3/4 and before the big line for 3 inches on a ruler?
3/8 of an inch in centimeter
That is 0.9525 cm