In more detail
Timing is measured in units: a dot is 1 unit, a dash 3, the gap inside a letter 1, between letters 3 and between words 7. E is a single dot and T a single dash, because they are the most common English letters. Speed is given in words per minute using the standard word PARIS, which is exactly 50 units including its word gap, so one unit lasts 1200 ÷ WPM milliseconds. See the Morse Code Cheat Sheet.
Why it matters
Decoding fails mostly on timing, not symbols: without clear letter and word gaps, ... followed by - (ST) is indistinguishable from ...- (V). The international (ITU) code used today differs from the original American Morse in several letters. Morse is still used by amateur radio operators, as aviation and navigation beacon identifiers, and as an accessibility input method.
Example
SOS is ... --- .... Counting units: S is 5, O is 11, plus two letter gaps of 3, gives 27 units. At 20 words per minute a unit is 1200 ÷ 20 = 60 ms, so the whole signal takes 1,620 ms. As a distress signal it is traditionally sent run together, without the letter gaps.