Watches that display time on a dial using rotating hands often have markings around the outside to make it easier to see the time. These are often called indices or hour markings and they can have a big impact on the aesthetic of a watch.
As the hour hand rotates every 12 hours and the minute/seconds hands every 60 minutes/seconds, you'd still be able to tell roughly what time it is if there were no markings on the dial. To make sure it's the right way round the crown and watch brand's logo will often indicate which way is up, where the top would be the 12th hour.
To make it easier to tell the time and to see it more accurately there are often 12 markings going along the outside of the dial that align with each hour of the hour hand/every 5 minutes of the minute hand. There are several common ways to display these markings, with for example numbers going from 1 to 12 for each hour, maybe symbols such as rectangles instead or a combination of both/something different.
Often watch indices refers to the hour markings on a dial, but some watches also have minute markings for every minute of the minute hand. This is less common, where when done the markings are typically smaller and often use small symbols near the edge of the dial. Sometimes you might even get Arabic number markings for the number of seconds that have passed next to the hour markings, where this would generally be smaller/less prominent than the hour markings.
Arabic Number Markers
Arabic numbers, also often called Arabic numerals, are commonly used for watch indices. An Arabic numeral is just a symbol used to display a decimal number, so what you'd know as the numbers 0 to 9, where Arabic numerals are the most widely used way of displaying decimals.
If you see a watch described as an Arabic number watch it would likely use these numbers 0 to 9, but the way they're used can vary. Some dials display the numbers for all 12 hours from 1 to 12, but there are many variations of this.
Some common designs might be for a dial to just display the 3rd, 6th ,9th and 12th hours with different markings in-between or none at all, or maybe to display these numbers larger or more prominently than the other hours. Alternatively, just the 6th and 12th hours might have numbers, or even just the 12th. The orientation may also vary, where the numbers are often displayed vertically, but they may also face towards the centre of the dial.
Roman Number Markers
Roman numbers, also often called Roman numerals, are less common than Arabic but are often used as watch indices. Roman numbers use different symbols to display a decimal number, where what you'd know as the number 1 is I, the number 5 is V and the number 10 is X.
Roman numbers would use combinations of their symbols for 1, 5 and 10 to make up all the numbers between 1 and 12. There are two methods used here; additive, where when you have a small number after a bigger number you add them together (e.g. XI would be 11) and subtractive, where when you have a small number before a big number you subtract it (e.g. IX would be 9).
This gets a bit confusing because you'd think the number 4 might use the subtractive method as it's shorter (IV), but often watches use the additive one of IIII for this instead and the subtractive for the number 9 (IX). If you're curious why watch dials do this it's worth doing your own research, where there are lots of potential reasons ranging from the subtractive method being newer to the symmetry of using IIII on the dial.
The most common usage of Roman numerals in watch indices for 1 to 12 is I, II, III, IIII, V, VI, VII, VIII, IX, X, XI, XII (with the IIII sometimes being IV instead). Similar to Arabic number markers, the design of a particular dial and orientation of the numbers can vary.
Symbol Markers
Instead of using Arabic/Roman numbers, symbols are also often used for watch indices. Shapes such as rectangles (wide rectangular indices are often called batons and thinner ones called sticks), triangles/arrows, circles and combinations of these and other shapes might be used.