The Sapphic stanza is composed of 4 lines, the first three lines consisting of 11 syllables, the last line of 5 syllables. The long lines are called hendecasyllabics, while the short line is called adonic. In their writing, the Greeks focused on long and short vowel sounds, today we focus on meter. Here is what the lines look like.
1 - two trochees, a dactyl, two trochees
2 - two trochees, a dactyl, two trochees
3 - two trochees, a dactyl, two trochees
4 - one dactyl, one trochee
What does this mean?
A trochee has two beats in the pattern stressed/unstressed, such as in words like happy, double, and planet. It is noted as / u.
A dactyl has three beats in the pattern stressed/unstressed/unstressed, such as in words like carefully, tenderly, and buffalo. It is note as / u u.
So using this notation, here's what a Sapphic stanza looks like metrically.
1 - / u / u / u u / u / u
2 - / u / u / u u / u / u
3 - / u / u / u u / u / u
4 - / u u / u
Originally, these stanzas were not rhymed, but in the Middle Ages they sometimes acquired the rhyme scheme abab.
Phew! That's a lot to remember. For more information, Poetry Magnum Opus has a terrific overview of the form and its changes through time. You can read some examples and learn more about the form in the the piece On Form: Rachel Wetzsteon.
Here's an example by the poet Sara Teasdale.
If I can bear your love like a lamp before me,
When I go down the long steep Road of Darkness,
I shall not fear the everlasting shadows,
Nor cry in terror.
If I can find out God, then I shall find Him,
If none can find Him, then I shall sleep soundly,
Knowing how well on earth your love sufficed me,
A lamp in darkness.