A dactylic foot (known as a dactyl) has a long syllable followed by two short syllables (LSS or /UU)
Tetrameter is four feet per line.
Part of the Beatles’ lyrics for Lucy in the Sky With Diamonds is a great example of dactylic tetrameter:
Picture yourself in a boat on a river with
Tangerine trees and marmalade skies
That this is not entirely dactylic – the last word of the second line (skies) is a single long syllable and, in the same line, “…trees and…” is a trochee (LS or /U). The first line, however, it pure dactylic tetrameter.
A closer look at the feet in these lines
U = short syllable; / = long syllable; | = division between feet