In Spanish, one syllable is always emphasised, and this stressed syllable is known as the sílaba tónica. To talk about where the sílaba tónica is located, you count back from the final syllable of the word. Let's look at this more carefully.
Stress falls on penultimate syllable
Many words in Spanish are stressed on the penultimate (second to last) syllable. These words are called "palabras llanas" (or sometimes "palabras graves").
When the word ends in -n, -s or a vowel, the word will generally be stressed on the penultimate syllable and does not need a written accent. Here are some examples:
This sentence divided into syllables:
As you can see, each word in this sentence is stressed on the penultimate syllable, has no written accent [´], and ends in -n or -s.
Here is another example:
See again how most words in the sentence are "llanas", stressed on the penultimate syllable, and have no written accent as they all end in -n, -s or a vowel.
If a word with these characteristics breaks the rule and is stressed on a different syllable, then we need a written accent, it tells us exactly which syllable needs to be stressed. Have a look:
We can see in the words "verán" and "sábado" where the stress goes thanks to the written accent on those syllables.
Stress falls on last syllable
Words that have the stress on the last syllable, e.g. "verán", are called "palabras agudas".
Stress falls on antepenultimate syllable
Words that have the stress on the antepenultimate (third to last) syllable, e.g. "sábado", are called "palabras esdrújulas". These words always have a written accent to show where the stress falls. Note that the word esdrújula is itself a "palabra esdrújula"!
Have a look at some other examples of words stressed on the last syllable:
And some examples of words stressed on the penultimate syllable:
And finally more examples of words stressed on the antepenultimate syllable: